Handle early blocks (#2155)

## Issue Addressed

NA

## Problem this PR addresses

There's an issue where Lighthouse is banning a lot of peers due to the following sequence of events:

1. Gossip block 0xabc arrives ~200ms early
    - It is propagated across the network, with respect to [`MAXIMUM_GOSSIP_CLOCK_DISPARITY`](https://github.com/ethereum/eth2.0-specs/blob/v1.0.0/specs/phase0/p2p-interface.md#why-is-there-maximum_gossip_clock_disparity-when-validating-slot-ranges-of-messages-in-gossip-subnets).
    - However, it is not imported to our database since the block is early.
2. Attestations for 0xabc arrive, but the block was not imported.
    - The peer that sent the attestation is down-voted.
        - Each unknown-block attestation causes a score loss of 1, the peer is banned at -100.
        - When the peer is on an attestation subnet there can be hundreds of attestations, so the peer is banned quickly (before the missed block can be obtained via rpc).

## Potential solutions

I can think of three solutions to this:

1. Wait for attestation-queuing (#635) to arrive and solve this.
    - Easy
    - Not immediate fix.
    - Whilst this would work, I don't think it's a perfect solution for this particular issue, rather (3) is better.
1. Allow importing blocks with a tolerance of `MAXIMUM_GOSSIP_CLOCK_DISPARITY`.
    - Easy
    - ~~I have implemented this, for now.~~
1. If a block is verified for gossip propagation (i.e., signature verified) and it's within `MAXIMUM_GOSSIP_CLOCK_DISPARITY`, then queue it to be processed at the start of the appropriate slot.
    - More difficult
    - Feels like the best solution, I will try to implement this.
    
    
**This PR takes approach (3).**

## Changes included

- Implement the `block_delay_queue`, based upon a [`DelayQueue`](https://docs.rs/tokio-util/0.6.3/tokio_util/time/delay_queue/struct.DelayQueue.html) which can store blocks until it's time to import them.
- Add a new `DelayedImportBlock` variant to the `beacon_processor::WorkEvent` enum to handle this new event.
- In the `BeaconProcessor`, refactor a `tokio::select!` to a struct with an explicit `Stream` implementation. I experienced some issues with `tokio::select!` in the block delay queue and I also found it hard to debug. I think this explicit implementation is nicer and functionally equivalent (apart from the fact that `tokio::select!` randomly chooses futures to poll, whereas now we're deterministic).
- Add a testing framework to the `beacon_processor` module that tests this new block delay logic. I also tested a handful of other operations in the beacon processor (attns, slashings, exits) since it was super easy to copy-pasta the code from the `http_api` tester.
    - To implement these tests I added the concept of an optional `work_journal_tx` to the `BeaconProcessor` which will spit out a log of events. I used this in the tests to ensure that things were happening as I expect.
    - The tests are a little racey, but it's hard to avoid that when testing timing-based code. If we see CI failures I can revise. I haven't observed *any* failures due to races on my machine or on CI yet.
    - To assist with testing I allowed for directly setting the time on the `ManualSlotClock`.
- I gave the `beacon_processor::Worker` a `Toolbox` for two reasons; (a) it avoids changing tons of function sigs when you want to pass a new object to the worker and (b) it seemed cute.

beacon_node/network/src/router/processor.rs

@@ -26,7 +26,7 @@ pub struct Processor<T: BeaconChainTypes> {
     /// A network context to return and handle RPC requests.
     network: HandlerNetworkContext<T::EthSpec>,
     /// A multi-threaded, non-blocking processor for applying messages to the beacon chain.
-    beacon_processor_send: mpsc::Sender<BeaconWorkEvent<T::EthSpec>>,
+    beacon_processor_send: mpsc::Sender<BeaconWorkEvent<T>>,
     /// The `RPCHandler` logger.
     log: slog::Logger,
 }
@@ -64,7 +64,7 @@ impl<T: BeaconChainTypes> Processor<T> {
             current_workers: 0,
             log: log.clone(),
         }
-        .spawn_manager(beacon_processor_receive);
+        .spawn_manager(beacon_processor_receive, None);
 
         Processor {
             chain: beacon_chain,
@@ -309,7 +309,7 @@ impl<T: BeaconChainTypes> Processor<T> {
         ))
     }
 
-    fn send_beacon_processor_work(&mut self, work: BeaconWorkEvent<T::EthSpec>) {
+    fn send_beacon_processor_work(&mut self, work: BeaconWorkEvent<T>) {
         self.beacon_processor_send
             .try_send(work)
             .unwrap_or_else(|e| {