Fix blocking messages thread

[mluts]

Hi, thank you for this repo, i'm glad that there is something like that in the wild.

While i was working on my plugin for vim i encountered a problem with messages thread. When i'm in the middle of accepting rpc request from nvim i can't do requests back to nvim because it will block messages thread forever.

So far these changes worked for me.

...And i think those handlers deserve their own functions

[jebberjeb]

Thanks @mluts, will take a look shortly.