dbus tools: print averages

dbus tools: print averages

This adds collecting and printing of average timings for
the dbus tools. Unfortunately, it is in a copy & paste style
as I didn't find a way to share BPF maps code between the tools.
I've even tried static implementation in an included header. Doesn't work.
Apparently bcc needs it in one file, analysed by bcc, before
it is passed to libllvm's preprocessor.

Change-Id: Id1c259b47ee7cfb0b05ac36d4800ea041af11e9c