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/main.rs

@@ -1,6 +1,5 @@
 #![cfg(not(debug_assertions))] // Tests are too slow in debug.
 
-pub mod common;
 pub mod fork_tests;
 pub mod interactive_tests;
 pub mod tests;