[FTS] rebalance progress bar jumps from 50% to done

1) define an FTS index.
2) then rebalance in another node.

Watch the progress bar, and you'll see it climb slowly up to 50%. The first 50% of progress represents the KV rebalancing phase (moving vbuckets amongst KV-engine (memcached/ep-engine) nodes).

The next 50-to-100% range of progress covers FTS index rebalancing.

In this phase, you'll see progress stuck at 50% for a long time (while underneath the hood, the FTS shards (or pindexes) are getting moved around.

Then, finally, when it's all done, progress suddenly snaps to 100% and suddenly it's all complete (assuming it doesn't error) and your eyes can catch that fast enough.

Instead, we should get granular progress climbing from 50% to 100% done.