## Issue Addressed
Closes#1680
## Proposed Changes
This PR fixes a race condition in beacon node start-up whereby the pubkey cache could be created by the beacon chain builder before the `PersistedBeaconChain` was stored to disk. When the node restarted, it would find the persisted chain missing, and attempt to start from scratch, creating a new pubkey cache in the process. This call to `ValidatorPubkeyCache::new` would fail if the file already existed (which it did). I changed the behaviour so that pubkey cache initialization now doesn't care whether there's a file already in existence (it's only a cache after all). Instead it will truncate and recreate the file in the race scenario described.
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