[BP of MB-17997 to 6.6.1] - [FTS] stats: UI: Percentage completion stat does not reflect updates

Build
4.5.0-1277

Testcase
./testrunner -i INI_FILE.ini -p skip-cleanup=True,cluster=D+F,GROUP=PS -t fts.stable_topology_fts.StableTopFTS.create_simple_default_index,items=10000,update=True,GROUP=PS

1. Load 10K keys into a bucket
2. Create index and wait till indexing completes
3. Update 30% of the keys
4. watch percentage completion stat. Stays at 100% although fts is just ingesting the updates from dcp.

Percentage completion should be based on remaining mutations and not keys to be indexed.