Improve redis-trib replica assignment

This tiny bit of code has gone through so many revisions.  Hopefully
it's more correct now.

Fixes #2204
Add 'age' value to SENTINEL INFO-CACHE
Merge pull request #2234 from mattsta/feature/sentinel-info-cache/age

Add 'age' value to SENTINEL INFO-CACHE
Merge pull request #2227 from mattsta/fix/trib/assignment/master-replica

Improve redis-trib replica assignment
INFO loading stats: three fixes.

1. Server unxtime may remain not updated while loading AOF, so ETA is
not updated correctly.

2. Number of processed byte was not initialized.

3. Possible division by zero condition (likely cause of issue #1932).
Merge branch 'unstable' of github.com:/antirez/redis into unstable