Convert data reading into event based system

Convert data reading into event based system

As of preparing for the future, convert the data reading into an even
based system. The main loop has got the loop that holds the state of
the logging process. All other modules then register itself into the
event system where they get called back whenever data gets feed in
into their file descriptors.

Signed-off-by: Timo Kokkonen <timo.t.kokkonen@iki.fi>