Modularise slasher backend (#3443)

## Proposed Changes

Enable multiple database backends for the slasher, either MDBX (default) or LMDB. The backend can be selected using `--slasher-backend={lmdb,mdbx}`.

## Additional Info

In order to abstract over the two library's different handling of database lifetimes I've used `Box::leak` to give the `Environment` type a `'static` lifetime. This was the only way I could think of using 100% safe code to construct a self-referential struct `SlasherDB`, where the `OpenDatabases` refers to the `Environment`. I think this is OK, as the `Environment` is expected to live for the life of the program, and both database engines leave the database in a consistent state after each write. The memory claimed for memory-mapping will be freed by the OS and appropriately flushed regardless of whether the `Environment` is actually dropped.

We are depending on two `sigp` forks of `libmdbx-rs` and `lmdb-rs`, to give us greater control over MDBX OS support and LMDB's version.

lighthouse/Cargo.toml

@@ -7,6 +7,7 @@ autotests = false
 rust-version = "1.62"
 
 [features]
+default = ["slasher-mdbx"]
 # Writes debugging .ssz files to /tmp during block processing.
 write_ssz_files = ["beacon_node/write_ssz_files"]
 # Compiles the BLS crypto code so that the binary is portable across machines.
@@ -19,6 +20,10 @@ milagro = ["bls/milagro"]
 spec-minimal = []
 # Support Gnosis spec and Gnosis Beacon Chain.
 gnosis = []
+# Support slasher MDBX backend.
+slasher-mdbx = ["slasher/mdbx"]
+# Support slasher LMDB backend.
+slasher-lmdb = ["slasher/lmdb"]
 
 [dependencies]
 beacon_node = { "path" = "../beacon_node" }
@@ -48,6 +53,7 @@ malloc_utils = { path = "../common/malloc_utils" }
 directory = { path = "../common/directory" }
 unused_port = { path = "../common/unused_port" }
 database_manager = { path = "../database_manager" }
+slasher = { path = "../slasher" }
 
 [dev-dependencies]
 tempfile = "3.1.0"

lighthouse/tests/beacon_node.rs

@@ -1288,6 +1288,32 @@ fn slasher_broadcast_flag() {
             assert!(slasher_config.broadcast);
         });
 }
+
+#[test]
+fn slasher_backend_default() {
+    CommandLineTest::new()
+        .flag("slasher", None)
+        .run_with_zero_port()
+        .with_config(|config| {
+            let slasher_config = config.slasher.as_ref().unwrap();
+            assert_eq!(slasher_config.backend, slasher::DatabaseBackend::Mdbx);
+        });
+}
+
+#[test]
+fn slasher_backend_override_to_default() {
+    // Hard to test this flag because all but one backend is disabled by default and the backend
+    // called "disabled" results in a panic.
+    CommandLineTest::new()
+        .flag("slasher", None)
+        .flag("slasher-backend", Some("mdbx"))
+        .run_with_zero_port()
+        .with_config(|config| {
+            let slasher_config = config.slasher.as_ref().unwrap();
+            assert_eq!(slasher_config.backend, slasher::DatabaseBackend::Mdbx);
+        });
+}
+
 #[test]
 pub fn malloc_tuning_flag() {
     CommandLineTest::new()