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>