[wip] implement socket pool
[wip] update the http1client example to use the socket pool
socket pool should be sharable bet. threads
change the URL of the test and see how it goes on travis-ci
Merge branch 'master' into kazuho/expr/proxy-keepalive

Conflicts:
	lib/socket.c
[refactor][wip] sockets that get associated to a pool should be created by the pool, and tracked by the pool
amend prev. commit (fix unused variable access)
don't do dup->close on export
add comment on libuv-side impl.
timeout should be an option of socket pool (and making it so would gives us the possibility to initialize the structure before creating a loop
it works
fix NULL access
turn down the keepalive flag if impossible to determine the end of the response
add config vars related to keepalive
close the downstream response when upstream closes with no data
respect proxy.timeout.keepalive
fix degrade leading to SEGV in commit bfcb80c
close the expired connections in pool whenever possible
no need to wait for lock in the timeout, now that the expired connections are closed whenever the lock is obtained
[refactor][test] no more hard-coded path in md5_file