KvStore - A data-store to support BlockTree (#2897)

* Mostly implement key-value store and add integration points

Essential key-value store functionality is implemented, needs more work to be integrated, tested, and activated.

Behind the `kvstore` feature.

core/src/lib.rs

@@ -39,6 +39,8 @@ pub mod fetch_stage;
 pub mod fullnode;
 pub mod gen_keys;
 pub mod gossip_service;
+#[cfg(feature = "kvstore")]
+pub mod kvstore;
 pub mod leader_confirmation_service;
 pub mod leader_schedule;
 pub mod leader_schedule_utils;