Maintain and report outgoing transfer statistics

Maintain and report outgoing transfer statistics

Transfer statistics are currently only reported for incoming transfers,
even though they are equally useful for outgoing transfers.  Define a
separate structure for keeping track of the number of messages, records,
and bytes sent during each outgoing transfer, along with the time each
outgoing transfer took.  Repurpose the 'nmsg' field of the xfrout_ctx_t
structure for tracking the number of messages actually sent, ensuring it
is only increased after isc_socket_send() indicates success.  Report the
statistics gathered when an outgoing transfer completes.