moved the http-related logic out of shardcached.c

now shcd_http.[ch] encapsulates mongoose and the whole http subsystem
updates to use the new mongoose API properly

we need to spawn server threads ourselves now ,
instructing them to use the same listening socket

(check https://github.com/cesanta/mongoose/blob/master/examples/multi_threaded.c for
 an example on how to use the mongoose api to serve requests in multiple threads)