ClusterInfo::process_packets handles incoming packets in a thread_pool:
https://github.com/solana-labs/solana/blob/87311cce7/core/src/cluster_info.rs#L2118-L2134
However, profiling runtime shows that threads are not well utilized and
a lot of the processing is done sequentially.
This commit redistributes the work done in parallel. Testing on a gce
cluster shows 20%+ improvement in processing gossip packets with much
smaller variations.
