As correctly pointed out by @rrhuffy, restricting update intervals to
minutes is pretty arbitrary. Therefore, change logic to specify updates
intervals in *seconds*.
Also, to maintain backwards compatibility for users that already have
custom intervals in their config, allow a module to specify their own
"factor" for intervals. So a module that expects the interval to be in
minutes can set the factor to 60.
Use generic interval mechanism in most of the modules that use slow
updates.
Only exception: getcrypto, as the interval is specified in seconds there
and I want to retain backwards-compatibility.
fixes#220
