Clarify CBL Replicator Progress

We are getting lots of confusion on Couchbase Lite's replicator progress numbers.

See CBSE 5862, for instance.

Some comments from that ticket:

 
Jens Alfke added a comment - 05/Oct/18 10:05 AM
The units are arbitrary. Exposing them directly in the API was a mistake; the only thing they should be used for is calculating a fraction.

(In practice, IIRC they are sometimes KB, sometimes doc counts when we don't yet know the size of the doc or whether we need to copy it at all.)
 
 added a comment - 05/Oct/18 10:14 AM
fraction of total ? But since the sync is in batches and the total count can vary through the course of a sync,  probably even that is not useful ?
 
Jens Alfke added a comment - 05/Oct/18 10:55 AM
I agree the progress fraction isn't super useful, but it's better than nothing, and maybe in the future we can make it more accurate somehow...