Change defaults for checksum-related API method keywords to False in
order to make behaviour and responsibility for the decision clearer.
API callers are now responsible for setting these flags to True where
they are needed.

Separate checksum retrieval, updates and verification more clearly.

Add a checkum keyword to DataObject::list.

Add checksum size/consistency check for zero-length data objects.

Adding this parameter allows a callable to be supplied which can
obtain per-file checksums during a recursive put operation. e.g. by
reading from an accompanying .md5 file or by deciding, based on file
size, whether to calculate checksums or to insist that an earlier
process supply one pre-calculated.

Improve test coverage of checksum operations.

This check make re-running tests more reliable in cases where the test
teardown has failed to remove test groups from iRODS.