Support "in place" updates for Cassandra and JDBC back ends

[sullivan-]

Implement Repo.inplace.updateByKeyVal and Repo.inplace.updateByQuery to do updates "in place", or on the database.

• See #43 for discussion of Repo.inplace
• Depends on #44, #46, and #47
• We may want to split this in two, but seeing as it's probably going to be a while before anyone gets to this, we'll leave it as a fat ticket for now
• See #45 for hints on the basic shape of the method signatures, implementation clues, and what kinds of integration tests will be necessary