plenty of memory optimizations and leak fix

v3.6

fix memory leak

v3.5

set inner recv chan length 1, emitter chan length to 1

v3.4

use unbuffered chan for better timing and memory

v3.3

recent ack ts

v3.2

use _itimediff for seqid compare

v3.1

fix fec seq wrap & insert seq caused by recent commits

v3.0

gosimple

limit only a fixed number of goroutines were created in kcp-go

standalone updater and emitter goroutine in the whole server-side system.

v2.20

adjust fastack threshold

v2.19

fix some mistakes for current changes