Why limit CutOverLockTimeoutSeconds to 10 seconds?

There's a command line flag (`-cut-over-lock-timeout-seconds`) which allows you to control how much time to wait for a lock when performing a cutover.

Why is this [limited to 10 seconds](https://github.com/github/gh-ost/blob/5294ab6bd10c36bba7cd37e5739e6cab70b2cfda/go/base/context.go#L341)? Seems like [MySQL defaults to 50 sec](https://dev.mysql.com/doc/refman/5.7/en/innodb-parameters.html#sysvar_innodb_lock_wait_timeout) and allows much larger values.

It would be nice for potentially high-load tables to increase this lock wait timeout. We've had to limit database load temporarily in order to perform cut-overs due to the short lock wait timeout.