[3.6.fb] dynamic disable_auto_compactions

Summary: Pulling diff from https://reviews.facebook.net/D24747

Test Plan:
export ROCKSDB_TESTS=Dynamic
./db_test

Reviewers: ljin, igor, sdong

Subscribers: dhruba, leveldb

Differential Revision: https://reviews.facebook.net/D27981
Merge branch '3.6.fb' of github.com:facebook/rocksdb into fix36-dyopt