This is a candidate for adding to libosmocore (as osmo_time_cc), but
let's first use this in osmo-bsc to make sure that it works as intended.
I started out expecting to be done with this in half an hour, but I
found out that accumulating elapsed time to an integer counter has a
staggering amount of complexity to it, and a million pitfalls.
The intended use is to report allAvailableSDCCHAllocated and
allAvailableTCHAllocated performance indicators in OsmoBSC. Hopefully
this will also be generally useful elsewhere, to be worth the effort.
Related: SYS#4878
Change-Id: Icdd36f27cb54b2e1b940c9e6404ba9dd3692a310
