Modularize beacon node backend (#4718)

#4669


  Modularize the beacon node backend to make it easier to add new database implementations

book/src/help_bn.md

@@ -11,6 +11,9 @@ Options:
       --auto-compact-db <auto-compact-db>
           Enable or disable automatic compaction of the database on
           finalization. [default: true]
+      --beacon-node-backend <DATABASE>
+          Set the database backend to be used by the beacon node. [possible
+          values: leveldb]
       --blob-prune-margin-epochs <EPOCHS>
           The margin for blob pruning in epochs. The oldest blobs are pruned up
           until data_availability_boundary - blob_prune_margin_epochs. [default:

book/src/installation-source.md

@@ -154,7 +154,7 @@ You can customise the features that Lighthouse is built with using the `FEATURES
 variable. E.g.
 
 ```
-FEATURES=gnosis,slasher-lmdb make
+FEATURES=gnosis,slasher-lmdb,beacon-node-leveldb make
 ```
 
 Commonly used features include:
@@ -163,11 +163,12 @@ Commonly used features include:
 - `portable`: the default feature as Lighthouse now uses runtime detection of hardware CPU features.
 - `slasher-lmdb`: support for the LMDB slasher backend. Enabled by default.
 - `slasher-mdbx`: support for the MDBX slasher backend.
+- `beacon-node-leveldb`: support for the leveldb backend. Enabled by default.
 - `jemalloc`: use [`jemalloc`][jemalloc] to allocate memory. Enabled by default on Linux and macOS.
   Not supported on Windows.
 - `spec-minimal`: support for the minimal preset (useful for testing).
 
-Default features (e.g. `slasher-lmdb`) may be opted out of using the `--no-default-features`
+Default features (e.g. `slasher-lmdb`, `beacon-node-leveldb`) may be opted out of using the `--no-default-features`
 argument for `cargo`, which can be plumbed in via the `CARGO_INSTALL_EXTRA_FLAGS` environment variable.
 E.g.