Previously, our Redis abstraction only worked with strings because it
only dealt with the somewhat nebulous concept of message metadata. Now
that we also want to use it for concrete types, it makes sense to bake
in automatic serialization from and deserialization to those types. This
frees consumers from the responsibility of having to serialize to a
Redis-friendly format manually.
This change achieves that aim by making the db methods generic. `db.set`
gets a type parameter that is `Serialize` and the getters get one that
is `DeserializeOwned`.

rfk approved these changes Oct 8, 2018