RPC: Implement an analogue for ch_logfile

[w0rp]

Vim 8.0 has a function called ch_logfile for logging all of the activity for channels and jobs to a file conveniently. This makes it easy to figure out where some interaction between some plugin and a program, especially a language server, might be going wrong. It would be very useful to have a similar function in NeoVim. Right now, I often advise users to try and repeat their bugs in Vim instead of NeoVim, so I can see the log file that ch_logfile creates.

[justinmk]

#7062 is related.

Nvim does have rich logging already for channels, but the depth (INFO, DEBUG, etc.) is controlled at compile-time. For channels I guess we need runtime controls.

[justinmk]

@w0rp could you post a sample of the log output, in particular what events do you expect to be logged?

[w0rp]

Here are some examples from users reporting ALE issues with ch_logfile.

dense-analysis/ale#1918 (comment)
dense-analysis/ale#1407 (comment)
dense-analysis/ale#1200 (comment)
dense-analysis/ale#1149 (comment)

The output shows jobs starting and stopping, channels opening and closing, messages that are sent back and forth, and so on. All of the above make spotting bugs pretty easy.