Preparatory patch for pre-computing binlog checksums outside of holding
LOCK_log.

The existing code for MYSQL_BIN_LOG::write_cache() was needlessly complex
and very hard to understand and modify for handling the new case where
pre-computed checksums are already present in the IO_CACHE.

Greatly simplify the logic by replacing the (implicit) state machine with
direct code that pulls the events one by one from the IO_CACHE. This removes
a lot of state flags and avoids duplicate code for handling full vs. split
headers.

This also removes the need for the CacheWriter class. As a bonus, this fixes
the bug that CacheWriter::write() was completely ignoring write errors. No
other functional changes are done with this patch, only code cleanup.
Reviewed-by: Monty <monty@mariadb.org>
Signed-off-by: Kristian Nielsen <knielsen@knielsen-hq.org>