Add execute mechanism to receiver nodes.

Add execute mechanism to receiver nodes.

Currently, receiver nodes have not much information to offer which could
be interesting for their child buffer tree nodes. This is about to change
with the introduction of the afh receiver and the para_play executable.
For example, para_play needs to query the number of chunks of the current
audio file from the afh receiver in order to display playback progress.

This patch adds only the new function pointer to struct receiver. The afh
receiver will be the first user of this.