MDEV-3861: Assertion in TC_LOG_MMAP.
MDEV-3861: Assertion in TC_LOG_MMAP.

Root cause was that number of entries in commit checkpoint buffer
was bigger than total available entries in the mmap()'ed score
file. This causes TC_LOG_MMAP to run out of entries before even
the first checkpoint is started, which causes a hang.

Fixed by making sure we have fewer entries within one commit
checkpoint than total available scorefile entries.

Another part of this bug was discovery of severel unrelated bugs
in TC_LOG_MMAP dating back to 5.1. These were fixed in 5.1 and
will be merged up (the problem this patch fixes exists only in
10.0).

ollie314/server,davidl-zend/zenddbi,flynn1973/mariadb-aix,natsys/mariadb_10.2,ollie314/server,ollie314/server,natsys/mariadb_10.2,natsys/mariadb_10.2,ollie314/server,flynn1973/mariadb-aix,flynn1973/mariadb-aix,natsys/mariadb_10.2,flynn1973/mariadb-aix,flynn1973/mariadb-aix,ollie314/server,natsys/mariadb_10.2,ollie314/server,davidl-zend/zenddbi,flynn1973/mariadb-aix,natsys/mariadb_10.2,ollie314/server,davidl-zend/zenddbi,davidl-zend/zenddbi,natsys/mariadb_10.2,natsys/mariadb_10.2,ollie314/server,davidl-zend/zenddbi,natsys/mariadb_10.2,davidl-zend/zenddbi,flynn1973/mariadb-aix,davidl-zend/zenddbi,ollie314/server,davidl-zend/zenddbi,ollie314/server,davidl-zend/zenddbi,flynn1973/mariadb-aix,natsys/mariadb_10.2,davidl-zend/zenddbi,flynn1973/mariadb-aix,ollie314/server,davidl-zend/zenddbi,flynn1973/mariadb-aix,flynn1973/mariadb-aix,natsys/mariadb_10.2