[GSI] coredumps collected in test_multi_create_query_explain_drop_index in bootstrap job

Steps:
1. Create and configure cluster
2. Create and load buckets
3. Create multiple indexes in async.
4. Run Explain queries in async along with kv mutations.
5. Verify the explain results.
6. Finally drop all indexes.

Testrunner log: http://qa.sc.couchbase.com/job/cen7-2i-set2-job2-bootstrap/256/consoleFull

Testrunner command: ./testrunner -i /tmp/centos_sdmb.ini gsi_type=memory_optimized,get-coredumps=True,get-cbcollect-info=True -t 2i.indexscans_2i.SecondaryIndexingScanTests.test_multi_create_query_explain_drop_index,groups=simple,dataset=default,use_gsi_for_primary=True,use_gsi_for_secondary=True,nodes_init=2,services_init=kv-kv:index:n1ql,standard_bucket=2,sasl_bucket=2

Coredumps are collected from this test.