Remove DB.Check(). Allow read-only Tx.Check().

This commit removes the DB.Check() function and instead makes the user decide
whether a transaction should be writable or read-only. Tx.Check() is not safe
to use concurrently on a read-only transaction, however, it significantly
improves the performance of it.
Merge pull request #174 from benbjohnson/remove-db-check

Remove DB.Check(). Allow read-only Tx.Check().