Summary:
Pull Request resolved: https://github.com/facebook/rocksdb/pull/15048
Adds a validated db_stress flag for selecting the WriteBufferManager flush policy and randomizes it in db_crashtest. Crash-test generation covers all three policies; kFlushLargestAcrossDBs automatically uses two DBs sharing one manager so the cross-DB path is exercised.
Reviewed By: pdillinger
Differential Revision: D114638570
fbshipit-source-id: 00f664445614e083a4c59d4e2bb0896731d4bea7