[cbq] generic index fails under load (err: backfill exceeded limit 5120)

When running queries on generic index (CREATE INDEX `gen` ON `bucket-1`((distinct (array `v` for `v` in pairs(self) end))) the query engine creates scan-backfill* files in /tmp but never deletes them. After few min of single-threaded load the cbq starts failing with 

backfill exceeded limit 5120  error message

having about 11k files in /tmp.

Query examples:

SELECT meta().id FROM `bucket-1` WHERE (`middle_name` >= "Evan")  LIMIT 100

SELECT meta().id FROM `bucket-1` WHERE (`gid` >= "481ea470-a3f8-5b05-8f80-f158c8ee34c3")  LIMIT 100

SELECT meta().id FROM `bucket-1` WHERE (`dob` between "2016-02-11" and "2016-08-28")  LIMIT 100,

 To run the load test:

1) ssh to 172.23.100.194

2) cd /root/alex/NAPA/YCSB

3) run 

./bin/ycsb run couchbase2 -P workloads/custom/workloadp -p couchbase.host=172.23.100.190 -p couchbase.bucket=bucket-1 -p couchbase.password=password -p operationcount=1000000