xdcr collection backfill performance improvement

Build : 7.0.0-3147

Observing severe performance drop in backfill pipeline performance . Below are the avg xdcr rate for backfill replication on two extreme cases . Replication rate is extremely slow (191 items/sec) with number of docs in initial replication 

Test 1 : 

• Create 1000 collections on both source and destination nodes
• Load 100M docs , 100000 docs on to each collections
• Create replication on first 999 collections (Initial replication )
• Wait till completion of docs (in 999 collections)
• edit existing replication link and add 1000th collection to it
• start monitoring items on destination cluster

http://perf.jenkins.couchbase.com/job/titan_collection/53/

Test 2 : 

• Create 1000 collections on both source and destination nodes
• Load 100M docs , 100000 docs on to each collections
• Create replication on first 1 collection (Initial replication )
• Wait till completion of docs (in 1 collection)
• edit existing replication link and add remaing 999 collection to it
• start monitoring items on destination cluster

http://perf.jenkins.couchbase.com/job/titan_collection/45/