[datapipe] Add enablers for logging datapipe value changes

A lot of mce functionality is based on pushing events through datapipes,
which then causes cascade of activity on other datapipes. Tracing this
can be cumbersome at times.

Add logging hooks to datapipe structure to allow generic value tracing,
and modify datapipe initialization so that data type specific hooks are
bound to all datapipes so that all datapipe activity can be tracked by
invoking mce with logging options such as:

  mce -T -ldatapipe.c:*_pipe

Also fix couple of cases where incorrect data type were used for datapipe
value tracking.

Signed-off-by: Simo Piiroinen <simo.piiroinen@jollamobile.com>