Allow compilation with no slasher backend (#3888)

## Proposed Changes

Allowing compiling without MDBX by running:

```bash
CARGO_INSTALL_EXTRA_FLAGS="--no-default-features" make
```

The reasons to do this are several:

- Save compilation time if the slasher won't be used
- Work around compilation errors in slasher backend dependencies (our pinned version of MDBX is currently not compiling on FreeBSD with certain compiler versions).

## Additional Info

When I opened this PR we were using resolver v1 which [doesn't disable default features in dependencies](https://doc.rust-lang.org/cargo/reference/features.html#resolver-version-2-command-line-flags), and `mdbx` is default for the `slasher` crate. Even after the resolver got changed to v2 in #3697 compiling with `--no-default-features` _still_ wasn't turning off the slasher crate's default features, so I added `default-features = false` in all the places we depend on it.

Co-authored-by: Michael Sproul <micsproul@gmail.com>

beacon_node/Cargo.toml

@@ -36,7 +36,7 @@ clap_utils = { path = "../common/clap_utils" }
 hyper = "0.14.4"
 lighthouse_version = { path = "../common/lighthouse_version" }
 hex = "0.4.2"
-slasher = { path = "../slasher" }
+slasher = { path = "../slasher", default-features = false }
 monitoring_api = { path = "../common/monitoring_api" }
 sensitive_url = { path = "../common/sensitive_url" }
 http_api = { path = "http_api" }