Fix cov reuse bug with DCA

Addresses issue with re-opening a coverage multiple times within a DCA session, i.e.:

	with DirectCoverageAccess() as dca:
		with dca.get_read_only_coverage(dataset_id) as cov:
			#do something with cov
			…

		with dca.get_editible_coverage(dataset_id) as cov:
			# do something with cov
			…

Adds test_dca_coverage_reuse to verify proper functionality