crimson/os/seastore: make the common write case the fast path

crimson/os/seastore: make the common write case the fast path

The goal of this commit is to move a common write path earlier
in the flow to improve performance by avoiding overheads such as
key removal and lookup costs.

To do so, this commit separates the common fast path from other
slower paths. This restructuring also makes the existing function
more straightforward by removing optional behavior controlled by flags.

Signed-off-by: Myoungwon Oh <ohmyoungwon@gmail.com>