crimson/osd: optimize crimson-osd's client requests process parallelism

crimson/osd: optimize crimson-osd's client requests process parallelism

Make client requests go to the concurrent pipeline stage "wait_repop" once they
are "submitted" to the underlying objectstore, which means their on-disk order
is guaranteed, so that successive client requests can go into the "process"
pipeline stage.

Signed-off-by: Xuehan Xu <xxhdx1985126@gmail.com>