Add beacon.watch (#3362)

> This is currently a WIP and all features are subject to alteration or removal at any time.

## Overview

The successor to #2873.

Contains the backbone of `beacon.watch` including syncing code, the initial API, and several core database tables.

See `watch/README.md` for more information, requirements and usage.

beacon_node/http_api/tests/interactive_tests.rs

@@ -1,11 +1,11 @@
 //! Generic tests that make use of the (newer) `InteractiveApiTester`
-use crate::common::*;
 use beacon_chain::{
     chain_config::ReOrgThreshold,
     test_utils::{AttestationStrategy, BlockStrategy, SyncCommitteeStrategy},
 };
 use eth2::types::DepositContractData;
 use execution_layer::{ForkchoiceState, PayloadAttributes};
+use http_api::test_utils::InteractiveTester;
 use parking_lot::Mutex;
 use slot_clock::SlotClock;
 use state_processing::{