Add support for rate log

[krichardsson]

A common thing to log is rates (events / second) and it would be nice to simplify this. Currently there are a number of implementations in various modules with counters, timers and so on to calculate rates. This could be simplified and should preferably be done in a similar way as when logging a variable.

The suggested solution is:

1. Add support to the logging framework to acquire log data through a function instead of reading a memory location.
2. Add support to the stats count module to simplify initializing, counting and logging rates