4.6.x backport - Rebalance gets stuck after cursor dropped

• Type: Technical task
• Resolution: Fixed
• Priority: Major
• Fix Version/s: 4.6.4
• Affects Version/s: 4.6.2
• Component/s: None
• Labels:

  None

For Gerrit Dashboard: MB-25798
#	Subject	Branch	Project	Status	CR	V
83519,3	[BP] MB-25798: Improved logging for handle slow stream & scheduling backfill	watson	ep-engine	Status: MERGED	+2	+1
83520,4	[BP] MB-25798: Log seqno data for when not scheduling backfill	watson	ep-engine	Status: MERGED	+2	+1
83521,4	[BP] MB-25798: Log when DcpProducer::Buffer log is full	watson	ep-engine	Status: MERGED	+2	+1
83524,3	[BP] MB-25798: Backfill task leave stream state unchanged	watson	ep-engine	Status: MERGED	+2	+1
83531,2	[BP] MB-25798: Re-register a dropped cursor if we don't backfill	watson	ep-engine	Status: MERGED	+2	+1
83533,2	[BP] MB-25798: Don't schedule backfill until previous is complete	watson	ep-engine	Status: MERGED	+2	+1
86927,1	Merge remote-tracking branch 'review/watson_ep' into kv_engine/spock	spock	kv_engine	Status: MERGED	+2	+1
87040,1	Merge remote-tracking branch 'couchbase/spock' into 'couchbase/master'	master	kv_engine	Status: MERGED	+2	+1