Change: move remaining schedule code to dedicated files

[mattmundell]

What

Move the rest of the schedules section out of manage_sql.c.

Why

Reducing size of manage_sql.c. Better per-resource file organisation.

References

Follows /pull/2887.

Testing

In GSA I modified a schedule, and checked that a task ran on schedule. I also compared GET_SCHEDULES with main:

o m m '<get_schedules tasks="1"/>' > /tmp/schedules-pr
o m m '<get_schedules tasks="1"/>' > /tmp/schedules-main
diff -u /tmp/schedules-main /tmp/schedules-pr