Fix a bug that doesn't release lock (panjf2000#79)

Update info and fix typos

Optimize loop queue of workers

Optimize the logic when putting worker back to sync.Pool

Add doc.go

Refine the logic of sync.Pool

Replace mutex with spin-lock

Update READMEs

worker: decRunning when revertWorker is false

Signed-off-by: Cholerae Hu <choleraehyq@gmail.com>

Remove some useless code