trx_t::rollback(): Renamed from trx_rollback_to_savepoint().

trx_t::rollback_low(): Renamed from trx_rollback_to_savepoint_low().

fts_sql_commit(): Defined as an alias of trx_commit_for_mysql().
fts_sql_rollback(): Defined as an alias of trx_t::rollback().

fts_rename_aux_tables_to_hex_format(): Fix the error handling
that likely never worked because we failed to roll back the
first transaction.