Fix crash recovery of key rotation

When MySQL 5.7.9 was merged to MariaDB 10.2.2, an important debug assertion was omitted from mlog_write_initial_log_record_low().

Some MariaDB encryption code is violating the contract that was changed by WL#7142: any mini-transaction that is going to modify a persistent tablespace must first register that tablespace by calling mtr_t::set_named_space(). Because the debug assertion on mtr_t::is_named_space() had been removed, the problem could only manifest itself by random failures of encryption crash recovery.

This started to occur more after the merge of MDEV-12253 from 10.1. The test encryption.innodb-redo-badkey was randomly emitting serious error messages on recovery:

2017-05-09 16:51:33 140058457458560 [ERROR] InnoDB: Missing MLOG_FILE_NAME or MLOG_FILE_DELETE before MLOG_CHECKPOINT for tablespace 1

The proper fix is to replace the missing debug assertion, to remove the improper suppressions of warnings, and to add the missing mtr_t::set_named_space() calls.