I'm investigating LWT performance with C* 3.11.3.
It looks that the performance is bounded by messaging latency when many
requests are issued concurrently.

According to the source code, the number of messaging threads per node is
only 1 thread for incoming and 1 thread for outbound "small" message to
another node.

I guess these threads are frequently interrupted because many threads are
executed when many requests are issued.
Especially, I think it affects the LWT performance when many LWT requests
which need lots of inter-node messaging are issued.

I measured that latency. It took 2.5 ms in average to enqueue a message at
a node and to receive the message at the **same** node with 96 concurrent
LWT writes.
Is it normal? I think it is too big latency, though a message was sent to
the same node.

Decreasing numbers of other threads like `concurrent_counter_writes`,
`concurrent_materialized_view_writes` reduced a bit the latency.
Can I change any other parameter to reduce the latency?
I've tried using message coalescing, but they didn't reduce that.

* Environment
- 3 node cluster
- Replication factor: 3
- Node instance: AWS EC2 i3.xlarge

* C* configuration
- Apache Cassandra 3.11.3
- commitlog_sync: batch
- concurrent_reads: 32 (default)
- concurrent_writes: 32 (default)

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