FTS: concurrent index partitions movement during rebalance seems to be slow.

Build: 7.0.0-5017

Scenario is the following:

Having cluster {kv} {fts}

I'm doing the following steps:

• set maxConcurrentPartitionMovesPerNode = 1
• create bucket `default`
• load 100,00 docs into it, docs are attached
• creating default fts index on `default` 
• set number of partitions to be 6 for fts index
• waiting for indexing finish
• start rebalance-in {kv,fts} node
• waiting for rebalance-in end, calculating rebalance time.
• rebalance {kv, fts} node out
• waiting for fts indexing finish
• set maxConcurrentPartitionMovesPerNode = 2
• rebalance in {kv, fts} node once again
• comparing 1'st rebalance time and 2'nd. 

It is not 100% reproducible, but sometimes, 2'nd time is longer than 1'st one.