feat(settlement-core): feature-gate backends_common for consumers

These are useful helper backend implementations for the Idempotent and Leftover stores, but there is no need to always export/build them

crates/interledger-settlement/Cargo.toml

@@ -39,4 +39,5 @@ rand = "0.7.2"
 
 [features]
 settlement_api = []
-redis = ["redis_crate"]
+backends_common = ["redis"]
+redis = ["redis_crate"]

crates/interledger-settlement/src/settlement_core/mod.rs

@@ -1,11 +1,13 @@
 /// The REST API
 pub mod api;
-/// Common backend utils for the IdempotentStore adn LeftoversStore traits
-pub mod backends_common;
 /// Expose useful utilities for implementing idempotent functionalities
 pub mod idempotency;
 /// Expose useful traits
 pub mod types;
+/// Common backend utils for the IdempotentStore and LeftoversStore traits
+/// Only exported if the `backends_common` feature flag is enabled
+#[cfg(feature = "backends_common")]
+pub mod backends_common;
 
 use num_bigint::BigUint;
 use num_traits::Zero;