replaced old "probabilistic" xdcr trace logging with log category

So that we can properly and reliably use this thing.

New log category has log level set to error by default. Causing all
log messages (logged with debug level) to not be logged.

When some specific issues needs to be investigated it's possible to
change log level at runtime to default.

E.g.:

wget -O- --post-data='ale:set_loglevel(xdcr_trace, debug).' \
   http://Administrator:asdasd@127.0.0.1:8091/diag/eval

Change-Id: I08aa3a3a3c72765af993de6e6047c11f7e8bdeca
Reviewed-on: http://review.couchbase.org/35275
Reviewed-by: Aliaksey Kandratsenka <alkondratenko@gmail.com>
Tested-by: Aliaksey Kandratsenka <alkondratenko@gmail.com>
MB-10457: added diagnostics for tracing xdcr data loss in ticket

It's not clear what is causing that data loss that's seen in
MB-10457. In order to investigate we need to log more.

Change-Id: I53d85dec34cbc488afc9dbb5ec944800f6e7ddee
Reviewed-on: http://review.couchbase.org/35276
Reviewed-by: Artem Stemkovski <artem@couchbase.com>
Reviewed-by: Aliaksey Artamonau <aliaksiej.artamonau@gmail.com>
Tested-by: Aliaksey Kandratsenka <alkondratenko@gmail.com>