Update xfers log format.

* NOTIFY message includes serial.
* Transfer start message includes serial(s).
* Transfer end message includes elapsed times.
* Each processed xfer delta is logged.

Examples:
Incoming IXFR of 'example.com.' with '<ip>' key 'abcd.': Started (serial 43).
Incoming IXFR of 'example.com.' with '<ip>' key 'abcd.': serial 43 -> 44.
Incoming IXFR of 'example.com.' with '<ip>' key 'abcd.': Finished in 0.10s (finalization 0.00s).
NOTIFY of 'example.com.' to '<ip>': Query issued (serial 44).
Outgoing AXFR of 'example.com.' to '<ip>' key 'abcd.': Started (serial 44).
Outgoing AXFR of 'example.com.' to '<ip>' key 'abcd.': Finished in 0.75s.

refs #2398

Change-Id: Ic4d0ca486dc3cf2c4616b77f359c037d53165f1d