Assertions failed due to incorrect handling of the --tc-heuristic-recover
option when InnoDB is in read-only mode either due to innodb_read_only=1
or innodb_force_recovery>3. InnoDB failed to refuse a XA COMMIT or
XA ROLLBACK operation, and there were errors in the error handling in
the upper layer.

This was fixed by making InnoDB XA operations respect the
high_level_read_only flag. The InnoDB part of the fix and
parts of the test main.tc_heuristic_recover were provided
by Marko Mäkelä.

LOCK_log mutex lock/unlock had to be added to fix MDEV-13438.
The measure is confirmed by mysql sources as well.

For testing of the conflicting option combination, mysql-test-run is
made to export a new $MYSQLD_LAST_CMD. It holds the very last value
generated by mtr.mysqld_start().  Even though the options have been
also always stored in $mysqld->{'started_opts'} there were no access
to them beyond the automatic server restart by mtr through the expect
file interface.

Effectively therefore $MYSQLD_LAST_CMD represents a more general
interface to $mysqld->{'started_opts'} which can be used in wider
scopes including server launch with incompatible options.

Notice another existing method to restart the server with incompatible
options relying on $MYSQLD_CMD is is aware of $mysqld->{'started_opts'}
(the actual options that the server is launched by mtr). In order to use
this method they would have to be provided manually.

NOTE: When merging to 10.2, the file search_pattern_in_file++.inc
should be replaced with the pre-existing search_pattern_in_file.inc.