Make client read and write under mutex lock

[vinistock]

Since the language server runs requests concurrently under threads, we should only read or write from the pipes under a mutex lock. Otherwise, we risk having mangled input/output, which will break the communication with the server.

[andyw8]

@vinistock was this in response to a particular behaviour you were seeing, or just out of caution?

[vinistock]

It might be related to the IncompleteMessage errors we are sometimes seeing. We have the same mutex synchronization in the Ruby LSP when writing/reading from pipes.