input/csv: expand cleanup (resource release) and unbreak file reload

The list of previously created channels is kept across file reloads. We
also need to keep (or re-create) the list of channels that are used for
datafeed submission of analog data.

Release more allocated resources in the .cleanup() routine, and do reset
internal state such that a .reset() thus .cleanup() then .receive() call
sequence will work. This code path is taken for re-import of files (see
bug #1241, CSV was affected).