Description of problem:

We need some feedback on the progress of a geo-rep a session.
Intuitively, one would like to know when the synchronization
completes. Alas, in real life use cases, changes are made
continuously, so a point of completion will never be reached.

However, it make sense to ask that given a certain timepoint,
when do we arrive to a state when all master side data has a
replica on slave that is no older than the timepoint.

Geo-rep checkpointing will be an UI enhancement that gives
an interface to ask and answer this question.

"Asking" is called "setting a checkpoint" which makes a note
of master's xtime at the given moment. When the above referred
sync state is reached, that we call the "completion of the
checkpoint" or "reaching the checkpoint".

"Answering" happens by providing info in geo-rep status
output about checkpoint completion.