Optimize pending attestation processing by adding batching (#15801)

* Optimize pending attestation processing by adding batching

* Update beacon-chain/sync/pending_attestations_queue.go

Co-authored-by: Radosław Kapka <[email protected]>

* Update beacon-chain/sync/pending_attestations_queue.go

Co-authored-by: Radosław Kapka <[email protected]>

* Add root for debug

* Change it to map

* Dont need receiver

* Use two slices

---------

Co-authored-by: Radosław Kapka <[email protected]>

Author: terencechain

beacon-chain/sync/BUILD.bazel

@@ -174,6 +174,7 @@ go_test(
         "fork_watcher_test.go",
         "kzg_batch_verifier_test.go",
         "once_test.go",
+        "pending_attestations_queue_bucket_test.go",
         "pending_attestations_queue_test.go",
         "pending_blocks_queue_test.go",
         "rate_limiter_test.go",