## Issue Addressed
Resolves#2611
## Proposed Changes
Adds a duplicate block root cache to the `BeaconProcessor`. Adds the block root to the cache before calling `process_gossip_block` and `process_rpc_block`. Since `process_rpc_block` is called only for single block lookups, we don't have to worry about batched block imports.
The block is imported from the source(gossip/rpc) that arrives first. The block that arrives second is not imported to avoid the db access issue.
There are 2 cases:
1. Block that arrives second is from rpc: In this case, we return an optimistic `BlockError::BlockIsAlreadyKnown` to sync.
2. Block that arrives second is from gossip: In this case, we only do gossip verification and forwarding but don't import the block into the the beacon chain.
## Additional info
Splits up `process_gossip_block` function to `process_gossip_unverified_block` and `process_gossip_verified_block`.
88063398f6