server: Add a check for a changing private key

This checks that the private key used to encode the passwords has not
changed since last successful startup, leading to a corruption of all
the passwords. Lots of common scenario are covered, with various
combinations of key in a file or from a seed, set in the config file or
in an env variable or through CLI, and so on.

server/Cargo.toml

@@ -25,6 +25,7 @@ base64 = "0.21"
 bincode = "1.3"
 cron = "*"
 derive_builder = "0.12"
+derive_more = "0.99"
 figment_file_provider_adapter = "0.1"
 futures = "*"
 futures-util = "*"
@@ -162,3 +163,7 @@ features = ["file_locks"]
 [dev-dependencies.uuid]
 version = "1"
 features = ["v4"]
+
+[dev-dependencies.figment]
+features = ["test"]
+version = "*"