yield thread while waiting for native json rpc

[blahgeek]

sit-for may not yield if there's input pending

it seems to fix emacs-lsp/emacs#12

update: previously I tried adding (thread-yield) before sit-for. that seems to fix the issue where the sending thread is starving, but the receive handler from run-with-timer may still be waiting. Now I try to use sleep-for instead of sit-for