Add rocksdb_allow_unsafe_alter to enable crash unsafe INPLACE ADD|DROP partition

The patch from Facebook MySQL 8 that implements INLPLACE ADD|DROP but in a crash unsafe way.
When we incorporate these into Percona Server, we will add a new option "rocksdb_allow_unsafe_alter" that enables this crash unsafe behavior (default disabled), putting the responsibility on the user to decide until such a time when atomic DDL is implemented.
It should be a CMDLINE/CNF only (readonly).