fix: delete tables from DDB on drop_all_tables (#2194)

Prior to this commit, issuing drop_all_tables on a listing database with
an external manifest store would delete physical tables but leave
references behind in the manifest store. The table drop would succeed,
but subsequent creation of a table with the same name would fail with a
conflict.

With this patch, the external manifest store is updated to account for
the dropped tables so that dropped table names can be reused.

python/Makefile

@@ -34,3 +34,7 @@ doctest:	## Run documentation tests.
 .PHONY: test
 test:		## Run tests.
 	pytest python/tests -vv --durations=10 -m "not slow and not s3_test"
+
+.PHONY: clean
+clean:
+	rm -rf data