MINOR: cache: Larger HTTP objects caching.

MINOR: cache: Larger HTTP objects caching.

This patch makes the capable of storing HTTP objects larger than a buffer.
It makes usage of the "block by block shared object allocation" new shctx API.

A new pointer to struct shared_block has been added to the cache applet
context to memorize the next block to be used by the HTTP cache I/O handler
http_cache_io_handler() to emit the data. Another member, named "sent" memorize
the number of bytes already sent by this handler. So, to send an object from cache,
http_cache_io_handler() must be called until "sent" counter reaches the size
of this object.