Prevent double import of blocks (#2647)

## 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`.

beacon_node/network/src/beacon_processor/tests.rs

@@ -201,6 +201,7 @@ impl TestRig {
             executor,
             max_workers: cmp::max(1, num_cpus::get()),
             current_workers: 0,
+            importing_blocks: Default::default(),
             log: log.clone(),
         }
         .spawn_manager(beacon_processor_rx, Some(work_journal_tx));