prepare for supporting header filter
nginx filter compile config
filter request interface
unify request & response headers for header filter (#55)  ,support
nested location s (#56), access handler (#53) and other fixs (#51 #52
#54)
optimize most well known mime type for content type
Unify temp bufferes for  thread pool mode & default & coroutine mode
because maybe user can invoke some api in their own  thread
optimization for request body (#57 ), coroutine & thread pool mode with
header filter (#55)
access remote service within header filter  under coroutine and thread
mode  ( #55 )
header filter for clojure ( #55 )
add more remote unit tests about #53 #54 #55 #56