Lsp sometimes breaks because of malformed server messages

[mpo-mjolner]

I have encountered an error where pylsp server sends malformed json. This breaks the lsp protocol session and causes any further client requests to be ignored.

I suspect it is caused by an issue in antlr parsing. We see examples of this in test outputs where characters occur in the output without good reason, mostly a number of dots and occasionally other characters.

[nneskildsf]

Closed in #728.

Merging this PR will fix the "malformed JSON" problem because it does not use multiprocessing for concurrency like standard python lsp does, see python-lsp/python-lsp-server#633.