Add new 'rate counter' implementation to libosmocore

Add new 'rate counter' implementation to libosmocore

A 'rate counter' is a counter that counts events but also keeps
track of the rate of events (per second, minute, hour and day).

'rate counters' are generally abstracted in 'rate counter groups',
which are instances of a 'rate counter group description'.  This
way we can have e.g. a description describing what kind of counters
a BTS (or TRX) has - and we can then create one instance of that
group for every BTS or TRX that exists.