idea: Move Concurrent logic out at Writer level and adding write_can_concurrent

[Xuanwo]

We are now repeating concurrent related logic many times. I feel like it's time for us to research how to move concurrent logic out as Writer level as we do for read. Also, we can add write_can_concurrent capability to make sure users can know if a service support concurrent write or not.

One possible idea is making oio::Write::write() returns a 'static future so users can Box::pin() it and join them while needed.

[Xuanwo]

This will be a breaking change. And I will propose an RFC after I figured out most details.