Signed-off-by: NSage Weil <sage@newdream.net>

Since we don't need to maintain large pools of messages, we can just
use the standard mempool_t.  We maintain a msgpool 'wrapper' because we
need the mempool_t* in the alloc function, and mempool gives us only
pool_data.
Signed-off-by: NSage Weil <sage@newdream.net>

Pass the front_len we need when pulling a message off a msgpool,
and WARN if it is greater than the pool's size.  Then try to
allocate a new message (to continue without failing).
Signed-off-by: NSage Weil <sage@newdream.net>

The msgpool is a basic mempool_t-like structure to preallocate
messages we expect to receive over the wire.  This ensures we have the
necessary memory preallocated to process replies to requests, or to
process unsolicited messages from various servers.
Signed-off-by: NSage Weil <sage@newdream.net>