Simplify the C++ interface by removing explicit epoch management.
This merge request depends on merge request !22 (merged).
Remove connection::current_epoch()/set_epoch() and move epoch management into the find/count_channels + get_avialability command.
For find/count_channels the channel_predicate object has been expanded and can now take an NDS::epoch parameter as well, allowing the epoch to be specified at the call point.
connection::get_availability has been expanded to take an NDS::epoch as the first parameter.
The SWIG interface has been maintained so that no changes to client code are needed. This was done by moving the logic for set_epoch/ current_epoch into the NDS_swig::connection object.