upgrading mongoose to last release

mongoose API changed ... and it's now single-threaded using asynchronous i/o
instead of creating a pool of workers to handle connections.

This means that shardcached needs to use the libshardcache asynchronous api and needs
to not block on shardcache_get()/shardcache_set() operations but instead spawn a thread
get/set the data and return it back to the mongoose i/o thread (main thread)
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)
commented out some assignments leading to "dead store"

reported by the static analyzer
'stuff' to have 'strptime()' imported when building on linux
stick to the stable (released) mongoose version 5.1

instead of using a random git clone (which reports version 5.2 but it's still a work-in-progress)

mongoose will be updated to 5.2 when it will be officially released
updated libshardcache

(a leak has been fixed there)