OPTIM: channel: speed up co_getline()'s search of the end of line

OPTIM: channel: speed up co_getline()'s search of the end of line

Previously, co_getline() was essentially used for occasional parsing
in peers's banner or Lua, so it could afford to read one character at
a time. However now it's also used on the TCP log path, where it can
consume up to 40% CPU as mentioned in GH issue #2731. Let's speed it
up by using memchr() to look for the LF, and copying the data at once
using memcpy().

Previously it would take 2.44s to consume 1 GB of log on a single
thread of a Core i7-8650U, now it takes 1.56s (-36%).