MDL-42084 Backup Increase timeout, add front-end server config

If there are remaining cases in the backup/restore code where
progress is not reported to the user during long-running tasks,
the backup timeout of 120 seconds may cause regressions over
Moodle 2.5. This change increases the timeout to one hour, which
was the previous timeout for the entire backup/restore, so that
we can be certain there are no regressions.

Systems that have a front-end server (for load balancing between
multiple Moodle servers) sometimes have a timeout in the few-minutes
range, meaning that they would not display/log a suitable PHP error
if this timeout should ever occur. I have added a configuration so
that Moodle now knows about the front-end server timeout and will
apply this restriction instead (if there is one).

This setting is placed in the 'HTTP' server configuration options
directly next to the other setting that you need if you have a
front-end server. It can potentially be used in other areas where
long timeouts may be a possibility.

Based on work by Eric Merrill.