* Simplify rpc encode/decode messages to rpc.send/rcp.receive
* Make missing handlers message throw a warning
* Clean up formatting style in log
* Move all non-RPC loop messages to trace instead of debug
* Add format func option to log to allow newlines in per log entry

The official developer documentation in in :h dev-lua-doc specifies to
use "--@" for special/magic tokens. However, this format is not
consistent with EmmyLua notation (used by some Lua language servers) nor
with the C version of the magic docstring tokens which use three comment
characters.

Further, the code base is currently split between usage of "--@",
"---@", and "--- @". In an effort to remain consistent, change all Lua
magic tokens to use "---@" and update the developer documentation
accordingly.

The `onexit` handler could set `message_callbacks` to `nil` within the
luv event loop while the mainloop runs a function that tries to access
`message_callbacks`.

This adds some checks to prevent errors in that case.

Fixes https://github.com/neovim/neovim/issues/14863

handlers passed to `lsp_buf_request` weren't called if the server
responded with an error that looks like this:

     "decoded", {
      error = {
        code = -32601,
        message = "No delegateCommandHandler for foo"
      },
      id = 5,
      jsonrpc = "2.0"
    }

An example where that happens is both eclipse.jdt.ls and the
haskell-language-server when invoking a command that doesn't exist:

    :lua vim.lsp.buf_request(
      0,
      'workspace/executeCommand',
      { command = 'foo' },
      function(err, _, res)
        print(vim.inspect(err), vim.inspect(res))
      end
    )

Some servers might respond to `workspace/executeCommand` requests with a
boolean result and that could be `false`.

A `false` result should be allowed and not trigger the `on_error`
handler:

    -- Invalid server message
    on_error(client_errors.INVALID_SERVER_MESSAGE, decoded)

Concrete example where this occurred is with eclipse.jdt.ls:

    vim.lsp.buf_request(
      0,
      'workspace/executeCommand',
      {
        command = 'java.project.isTestFile',
        arguments = { vim.uri_from_bufnr(0), },
      },
      function(err, _, resp)
        print(vim.inspect(err), vim.inspect(resp))
      end
    )

Breaking Changes:
- Deprecated all `vim.lsp.util.{*diagnostics*}()` functions.
    - Instead, all functions must be found in vim.lsp.diagnostic
    - For now, they issue a warning ONCE per neovim session. In a
      "little while" we will remove them completely.
- `vim.lsp.callbacks` has moved to `vim.lsp.handlers`.
    - For a "little while" we will just redirect `vim.lsp.callbacks` to
      `vim.lsp.handlers`. However, we will remove this at some point, so
      it is recommended that you change all of your references to
      `callbacks` into `handlers`.
    - This also means that for functions like |vim.lsp.start_client()|
      and similar, keyword style arguments have moved from "callbacks"
      to "handlers". Once again, these are currently being forward, but
      will cease to be forwarded in a "little while".
- Changed the highlight groups for LspDiagnostic highlight as they were
  inconsistently named.
    - For more information, see |lsp-highlight-diagnostics|
- Changed the sign group names as well, to be consistent with
  |lsp-highlight-diagnostics|

General Enhancements:
- Rewrote much of the getting started help document for lsp. It also
  provides a much nicer configuration strategy, so as to not recommend
  globally overwriting builtin neovim mappings.

LSP Enhancements:
- Introduced the concept of |lsp-handlers| which will allow much better
  customization for users without having to copy & paste entire files /
  functions / etc.

Diagnostic Enhancements:
- "goto next diagnostic" |vim.lsp.diagnostic.goto_next()|
- "goto prev diagnostic" |vim.lsp.diagnostic.goto_prev()|
    - For each of the gotos, auto open diagnostics is available as a
      configuration option
- Configurable diagnostic handling:
    - See |vim.lsp.diagnostic.on_publish_diagnostics()|
    - Delay display until after insert mode
    - Configure signs
    - Configure virtual text
    - Configure underline
- Set the location list with the buffers diagnostics.
    - See |vim.lsp.diagnostic.set_loclist()|
- Better performance for getting counts and line diagnostics
    - They are now cached on save, to enhance lookups.
    - Particularly useful for checking in statusline, etc.
- Actual testing :)
    - See ./test/functional/plugin/lsp/diagnostic_spec.lua
- Added `guisp` for underline highlighting

NOTE: "a little while" means enough time to feel like most plugins and
plugin authors have had a chance to refactor their code to use the
updated calls. Then we will remove them completely. There is no need to
keep them, because we don't have any released version of neovim that
exposes these APIs. I'm trying to be nice to people following HEAD :)

Co-authored: [Twitch Chat 2020](https://twitch.tv/teej_dv)