Framework statistics

# Framework statistics

Implement some basic statistics for the framework (mainly service registry) to retrieve at least the following info:
- Nr of service registered
- Nr of service unregistered
- Average properties size (entries + kb)
- Max, min properties size and maybe std dev
- Nr of service on demand created
- Uptime
- Nr of bundles installed, unintstalled, stopped, started
- Maybe Nr of component created, start/stop cycles, etc

This should disclosed through be a Apache Celix internal framework API, but can be used in a standard shell command so that a `celix::stats` command can be made.