feat: heed db backend (LMDB)

Cargo.toml

@@ -79,12 +79,14 @@ parking_lot = { version = "0.11.1", optional = true }
 crossbeam = { version = "0.8.1", optional = true }
 num_cpus = "1.13.0"
 threadpool = "1.8.1"
+heed = { git = "https://github.com/Kerollmops/heed.git", rev = "b235e9c3e9984737c967b5de1014b48f125dc28b", optional = true }
 
 [features]
-default = ["conduit_bin", "backend_sqlite"]
+default = ["conduit_bin", "backend_heed"]
 backend_sled = ["sled"]
 backend_rocksdb = ["rocksdb"]
 backend_sqlite = ["sqlite"]
+backend_heed = ["heed", "crossbeam"]
 sqlite = ["rusqlite", "parking_lot", "crossbeam", "tokio/signal"]
 conduit_bin = [] # TODO: add rocket to this when it is optional