Clean up ValidatorDir lockfiles

[michaelsproul]

Description

The lighthouse account manager uses different code from the validator client to do its locking, and previous to #1958 they were completely out of sync. For now I'm making it so that the account manager locks the keystore at the hardcoded path it expects (voting-keystore.json.lock), so that if another process has locked this keystore, it fails. But this isn't ideal, because keystores aren't always named as the ValidatorDir code expects.