do not parse all the incoming requests at once, but delay them when the number exceeds http2_max_concurrent_requests_per_connection
accept as HTTP/2 if the request starts with "PRI * HTTP/2" and if http1-upgrade-to-http2 is set
Merge branch 'kazuho/http2/optimize-locality'
call read_start/read_stop only when necessary (and do the checks whether or not to stop parsing within `parse_input` (10% faster for 4K response)
add configuration directive:`max-connections`, observe the number of connections, and start / stop accepting new connections, with using signals to notify other threads
Merge pull request #23 from kazuho/kazuho/expr/max-connections

implement max_connections
http1 conn should not call `close_cb` in case the connection was stolen (fix degrade in 5c87c79)
fix HTTP/2 encode error in case the status code was not one of 200,204,206,304,400,404,500
[wip] compress response using indices
optimize the loop (a bit)
limit the number of entries in output header table to 32 (so that table compression should not slow down too much)
add NULL check in case the header is too long to be stored in the header table
add missing guard
remove `value_is_const`
remove `name_is_token`
nit-pick hpack impl. for simplicity (merge branch 'kazuho/refactor/hpack')
Merge branch 'master' into kazuho/expr/hpack-using-index

Conflicts:
	lib/http2/hpack.c
shrink the size of the table to max-1, before adding a new entry
record and iterate last 32 headers for index-based header compression (merge branch 'kazuho/expr/hpack-using-index') (close #22)
send Date and Server headers in HTTP2