Make PURGE BINARY LOGS more flexible

Right now PURGE BINARY LOGS has two modes:

• purge logs before a specific date
• purge logs older than a given binlog file name

For some situations it would also be convenient to have:

• purge logs and only keep N latest files
• purge logs if sum of binlog file sizes is beyond given size threshold

E.g.

PURGE BINARY LOGS KEEP LATEST 20;

PURGE BINARY LOGS BEYOND 100GB;

or similar.