Shift HTTP server heavy-lifting to blocking executor (#1518)

## Issue Addressed

NA

## Proposed Changes

Shift practically all HTTP endpoint handlers to the blocking executor (some very light tasks are left on the core executor).

## Additional Info

This PR covers the `rest_api` which will soon be refactored to suit the standard API. As such, I've cut a few corners and left some existing issues open in this patch. What I have done here should leave the API in state that is not necessary *exactly* the same, but good enough for us to run validators with. Specifically, the number of blocking workers that can be spawned is unbounded and I have not implemented a queue; this will need to be fixed when we implement the standard API.

beacon_node/rest_api/tests/test.rs

@@ -804,7 +804,7 @@ fn get_version() {
     let version = env
         .runtime()
         .block_on(remote_node.http.node().get_version())
-        .expect("should fetch eth2 config from http api");
+        .expect("should fetch version from http api");
 
     assert_eq!(
         lighthouse_version::version_with_platform(),