PG,ReplicatedPG: improve object_context snap_context debugging

Verify on on_activate() that snap_contexts and object_contexts
are empty.  Also, print out ref count changes for snap contexts.

MightReveal: 7398
Signed-off-by: Samuel Just <sam.just@inktank.com>
Merge remote-tracking branch 'origin/wip-7398' into wip-sam-testing