Introduce multiqueue_process_priority() to process only events at or
above a certain priority.

Asynchronous API functions are served immediately, which means pending
input could change the state of Nvim shortly after an async API function
result is returned.

nvim_get_mode() is different:
  - If RPCs are known to be blocked, it responds immediately (without
    flushing the input/event queue)
  - else it is handled just-in-time before waiting for input, after
    pending input was processed. This makes the result more reliable
    (but not perfect).

Internally this is handled as a special case, but _semantically_ nothing
has changed: API users never know when input flushes, so this internal
special-case doesn't violate that. As far as API users are concerned,
nvim_get_mode() is just another asynchronous API function.

In all cases nvim_get_mode() never blocks for more than the time it
takes to flush the input/event queue (~µs).

Note: This doesn't address #6166; nvim_get_mode() will provoke #6166 if
e.g. `d` is operator-pending.

Closes #6159

Also re-word some error messages:
- "Key does not exist: %s"
- "Invalid channel: %<PRIu64>"
- "Request array size must be 4 (request) or 3 (notification)"
- "String cannot contain newlines"

References #6150

Hope this will make people using feed_command less likely: this hides bugs.
Already found at least two:

1. msgpackparse() will show internal error: hash_add() in case of duplicate
   keys, though it will still work correctly. Currently silenced.
2. ttimeoutlen was spelled incorrectly, resulting in option not being set when
   expected. Test was still functioning somehow though. Currently fixed.

- nvim_get_option should return the global default of a local option.
- nvim_set_option should set the global default of a local option.

When the buffer that nvim_buf_set_lines() is changing is not in any vim
window, fix_cursor() leads to calling ml_get_buf() with an invalid line
number. The condition that fix_cursor() was called on was (buf ==
curbuf), but this is always true because of the call to
switch_to_win_for_buf() earlier in the function.

Instead this should be predicated on (save_curbuf.br_buf == NULL)