The master_secret setting is a `Vec<u8>`, for which there is no default
deserialization. This change pulls it out to a struct and implements
Deserialize so that it can be set with a string via environment variable
or config file.
That change also represented an opportunity to cache the value of
signing_secret, which was being freshly calculated on every request
even though the value is a constant derived from master_secret. So the
struct is actually called `Secrets` and has two properties.
This does leave a slightly weird naming mismatch that I wasn't sure what
to do with. `master_secret` makes sense from the perspective of setting
e.g. a `SYNC_MASTER_SECRET` environment variable, but it looks kind of
funny when the code accesses that property internally as a `Secrets`
struct with its own `master_secret` and `signing_secret` properties. I
tried to minimise the weirdness by passing around the `Secrets` instance
and elsewhere referring to that variable/argument as `secrets`.