Compute the maximal chunk size only once.

This patch adds the new field max_chunk_size to struct audio_file_data.
It is initialized in the afs process when the chunk table is saved,
just before the audio_file_data struct is passed to the server process.

The vss code benefits from this change since it can just use the
new information rather than calling afh_get_largest_chunk_size()
to recompute the maximal chunk size from scratch whenever a new
client connects.

Since vss.c was the only user of afh_get_largest_chunk_size(), we
may kill this function.