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.

lighthouse/environment/src/executor.rs

@@ -8,7 +8,7 @@ use tokio::runtime::Handle;
 #[derive(Clone)]
 pub struct TaskExecutor {
     /// The handle to the runtime on which tasks are spawned
-    pub(crate) handle: Handle,
+    pub handle: Handle,
     /// The receiver exit future which on receiving shuts down the task
     pub(crate) exit: exit_future::Exit,
     /// Sender given to tasks, so that if they encounter a state in which execution cannot