MessagePack RPC improvements.

We migrated away from gettimeofday in https://github.com/neovim/neovim/pull/975. We provide `os_hrtime()` instead:

https://github.com/aktau/neovim/commit/fb5a786bdb5b7b52b9c36b3eb8b6d2cc002aa8f3#diff-1a32cc1056b083197e9462b926958276R3954