MB-100: dump trace when vb replicator starts

Today we only dump traces after vb rep gets the token from throttle.
For easier diag of restart problem in future, add a trace at the very
beginning of vb rep starts.

Change-Id: Ib1fc74af5f3adb9fa30ede3736694ebffb4153a7
Signed-off-by: Aliaksey Kandratsenka <alk@tut.by>
Reviewed-on: http://review.couchbase.org/34141
Reviewed-by: Aliaksey Kandratsenka <alkondratenko@gmail.com>
Tested-by: Aliaksey Kandratsenka <alkondratenko@gmail.com>