[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
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)