WL#1362: Various sync modes for MySQL to improve reliability

Affects: Server-7.1   —   Status: Un-Assigned   —   Priority: Very High

This is entered based on Stefan Hinz request, which he passed on behalf from
some users.

MySQL has a lot of log files, some of them are rather critical (binary logs),
others may be critical in some configurations (ie used for auditing).
It would be good to have some option similar to innodb_flush_logs_at_trx_commit,
which would allow to configure flushing behavior for various log files. 

There shall be options for normal write (to OS cache) and sync (to disk)

Note this also applies to MySQL system files, such as master.info  - some users
would like extra safety having it actually hiting the disk, while others (ie
replicating to in-memory tables) would instead like to avoid flushing it for
each statement at all.