opt: put the static buffer back to pool to reduce more memory usages

chore: update some comments

chore: resolve the lint issue of staticcheck

bug: set the status of ring-buffer to non-empty after it grows its size

Fixes panjf2000#288

opt: improve the management logic of the mixed-buffer

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