MB-10057: expose testrunner api that returns json of checkpoints

I.e. so that checkpoints can be verified from tests

Change-Id: I5011229de5c5e94d425fcab662a3bf4a9294fd51
Reviewed-on: http://review.couchbase.org/34071
Tested-by: Aliaksey Kandratsenka <alkondratenko@gmail.com>
Reviewed-by: Aliaksey Kandratsenka <alkondratenko@gmail.com>
MB-10057: added bucket uuid to remote checkpointing payload

It's better if we can detect speaking to wrong bucket asap.

Change-Id: I39492992c97ac9b3b6c50a58fc8eeeb638fa3d49
Reviewed-on: http://review.couchbase.org/34072
Tested-by: Aliaksey Kandratsenka <alkondratenko@gmail.com>
Reviewed-by: Aliaksey Kandratsenka <alkondratenko@gmail.com>