Fix race conditions with proof state updates.

Add a strict set of updates to prevent incorrect state changes and correct
usage. Supporting the transaction at the trait level prevented some cases, but
having a strict set of state change flows is better.

This bug was found while developing the signatory. The keys are read from
memory, triggering race conditions at the database, and some `Pending` states
are selected (instead of just selecting `Unspent`).

This PR also introduces a set of generic database tests to be executed for all
database implementations, this test suite will make sure writing and
maintaining new database drivers

crates/cdk-common/src/database/mod.rs

@@ -1,7 +1,7 @@
 //! CDK Database
 
 #[cfg(feature = "mint")]
-mod mint;
+pub mod mint;
 #[cfg(feature = "wallet")]
 mod wallet;
 
@@ -16,6 +16,8 @@ pub use mint::{
 #[cfg(feature = "wallet")]
 pub use wallet::Database as WalletDatabase;
 
+use crate::state;
+
 /// CDK_database error
 #[derive(Debug, thiserror::Error)]
 pub enum Error {
@@ -53,4 +55,16 @@ pub enum Error {
     /// Invalid keyset
     #[error("Unknown or invalid keyset")]
     InvalidKeysetId,
+    /// Invalid state transition
+    #[error("Invalid state transition")]
+    InvalidStateTransition(state::Error),
+}
+
+impl From<state::Error> for Error {
+    fn from(state: state::Error) -> Self {
+        match state {
+            state::Error::AlreadySpent => Error::AttemptUpdateSpentProof,
+            _ => Error::InvalidStateTransition(state),
+        }
+    }
 }