Support the concurrent request.

[lsk569937453]

1. Add the concurrency examples for pure Grpc client and Qdrant client.
2. Remove the Rwlock of Channel as the concurrency request could use the same channel.
3. Remove the allow_retry param for with_channel as the tonic will retry automatically.

[agourlay]

Thanks for your contribution!

Could you please elaborate regarding the motivation for this change?

Also it would be easier to review if the clippy fixes were handled in a different PR.

[lsk569937453]

Hello, I want to join the company of Qdrant. I have sent the cv. And Andre said that I have to make a contribution to the https://github.com/qdrant/rust-client. And I get this issue(#30) in the github issues.
For the concurrent use of the client, I get hyperium/tonic#285 maybe we do not need the pool to do the concurrent request. We could just clone the client to do the concurrent request.
I don't understand which style we use.I just format it using rust-clippy.