Summary:
Fixes T8781168.

Added a new function EnableAutoCompactions in db.h to be publicly
avialable.  This allows compaction to be re-enabled after disabling it via
SetOptions

Refactored code to set the dbptr earlier on in TransactionDB::Open and DB::Open
Temporarily disable auto_compaction in TransactionDB::Open until dbptr is set to
prevent race condition.

Test Plan:
Ran make all check

verified fix on myrocks side:
was able to reproduce the seg fault with
../tools/mysqltest.sh --mem --force rocksdb.drop_table

method was to manually sleep the thread after DB::Open but before TransactionDB ptr was
assigned in transaction_db_impl.cc:
  DB::Open(db_options, dbname, column_families_copy, handles, &db);
  clock_t goal = (60000 * 10) + clock();
  while (goal > clock());
  ...dbptr(aka rdb) gets assigned below

verified my changes fixed the issue.

Also added unit test 'ToggleAutoCompaction' in transaction_test.cc

Reviewers: hermanlee4, anthony

Reviewed By: anthony

Subscribers: alex, dhruba

Differential Revision: https://reviews.facebook.net/D51147