* stable-2.15:
Bazel: Enable Error-Prone by default
Parallelize account migration in schema 146 migration
Fix _nextStepHandle not being assigned a value when scrolling
Elasticsearch: Remove redundant handling of 'ignore_unmapped' property
Elasticsearch: Fix support for V6 versions earlier than 6.7.*
ElasticVersionTest: Add missing assertions on V6_7
Change-Id: Ib655c541a28e3a9a2197465a58e1eff12123c6a5