Add parallel writes test.

[sunli829]

Hey, I am here again. 🙂

I found a small problem. When I call client_write in parallel, it will block and keep outputting the following log.

ERROR async_raft::replication: timeout while sending AppendEntries RPC to target error=deadline has elapsed
ERROR async_raft::replication: timeout while sending AppendEntries RPC to target error=deadline has elapsed
ERROR async_raft::replication: timeout while sending AppendEntries RPC to target error=deadline has elapsed
ERROR async_raft::replication: timeout while sending AppendEntries RPC to target error=deadline has elapsed
ERROR async_raft::replication: timeout while sending AppendEntries RPC to target error=deadline has elapsed

So I opened this PR to reproduce the problem.

[sunli829]

After further testing, I found that this problem is actually not related to parallel writing. AppendEntries takes too long, and this problem will definitely occur. hope it is of help to you. 😁

[MarinPostma]

@sunli829 could this be related to this? #76

[thedodd]

@MarinPostma I think you are correct. I definitely need to prioritize getting that issue addressed.