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.

.github/workflows/test-suite.yml

@@ -102,6 +102,16 @@ jobs:
       run: rustup update stable
     - name: Run operation_pool tests for all known forks
       run: make test-op-pool
+  slasher-tests:
+    name: slasher-tests
+    runs-on: ubuntu-latest
+    needs: cargo-fmt
+    steps:
+    - uses: actions/checkout@v1
+    - name: Get latest version of stable Rust
+      run: rustup update stable
+    - name: Run slasher tests for all supported backends
+      run: make test-slasher
   debug-tests-ubuntu:
     name: debug-tests-ubuntu
     runs-on: ubuntu-22.04