REORG: stkctr: move all the stick counters processing to stick-tables.c

REORG: stkctr: move all the stick counters processing to stick-tables.c

Historically we used to have the stick counters processing put into
session.c which became stream.c. But a big part of it is now in
stick-table.c (eg: converters) but despite this we still have all
the sample fetch functions in stream.c

These parts do not depend on the stream anymore, so let's move the
remaining chunks to stick-table.c and have cleaner files.

What remains in stream.c is everything needed to attach/detach
trackers to the stream and to update the counters while the stream
is being processed.