refactor: curp client propose

[bsbds]

This PR includes several changes:

• Moves client propose code to a seperate module propose_impl
• Improve propose latency for mutative commands This is done by simultaneously waitting for the record request to the cluster and the message from the leader gRPC stream.

Please briefly answer these questions:

• what problem are you trying to solve? (or if there's no problem, what's the motivation for this change?)

• what changes does this pull request make?

• are there any non-obvious implications of these changes? (does it break compatibility with previous versions, etc)