chore: move the ownerships of reactors from server to eventloop

Introduce poll_opt build tag and optimize the growth strategy of ring…

…-buffer

fix: don't shrink ring-buffer when it's empty

Fixes panjf2000#226

chore: sort imports in internal/socket

fix: resolve the contention issue in internal logger

fix: resolve the contention issue in internal logger

trifle: remove redundant test code

fix: prevent servers from panicking reads in closed sockets

Fixed panjf2000#194

opt: improve the logic of preventing server blocking

trifle: refine ReadBufferCap option and add comments