Log block import source (#5738)

* the default target peers is 100

* add some comments

* Merge branch 'unstable' of https://github.com/sigp/lighthouse into track-block-import-source

* add block import source

* revert

* update logging text

* fix tests

* lint

* use % instaed of to_string

beacon_node/beacon_chain/src/test_utils.rs

@@ -1881,6 +1881,7 @@ where
                 block_root,
                 RpcBlock::new(Some(block_root), block, sidecars).unwrap(),
                 NotifyExecutionLayer::Yes,
+                BlockImportSource::RangeSync,
                 || Ok(()),
             )
             .await?
@@ -1907,6 +1908,7 @@ where
                 block_root,
                 RpcBlock::new(Some(block_root), block, sidecars).unwrap(),
                 NotifyExecutionLayer::Yes,
+                BlockImportSource::RangeSync,
                 || Ok(()),
             )
             .await?