Made long semaphore wait timeout configurable

https://github.com/facebook/mysql-5.6/commit/cd063ab930f05efdba39d504543998512d1bd71f

This diff introduced a new system variable fatal_semaphore_timeout,
it makes the fatal semaphore timeout configurable.

Its default/min/max vlaues are 600/1/2^32-1 in seconds (it was hardcoded
as 600, now its default value is 600, so the default behavior of this diff
should be no change).