for bug #194, disable the srs fd poll.
for bug #194, remove the poll, it's no-use
fix #194: refine the timeout recv, pulse to 500ms. 2.0.14.
add comments about bug #196
add comments about bug #197
for bug #199, refine the api of send message.
refine code, rename the c0c3 header cache and iov cache.
for bug #199, refine the header generate
fix #194, writev multiple msgs, support 6k+ 250kbps clients. 2.0.15.
refine code for bug #194, add comments to macros.