## Issue Addressed
Closes#1787
## Proposed Changes
* Abstract the `ValidatorPubkeyCache` over a "backing" which is either a file (legacy), or the database.
* Implement a migration from schema v2 to schema v3, whereby the contents of the cache file are copied to the DB, and then the file is deleted. The next release to include this change must be a minor version bump, and we will need to warn users of the inability to downgrade (this is our first DB schema change since mainnet genesis).
* Move the schema migration code from the `store` crate into the `beacon_chain` crate so that it can access the datadir and the `ValidatorPubkeyCache`, etc. It gets injected back into the `store` via a closure (similar to what we do in fork choice).
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