crimson/osd: coroutinize OSD::start()

OSD::start() is a long, deeply nested .then() continuation chain. let's
rewrite it as a coroutine to make it readable. the chain was already
sequential and the one concurrent step keeps its when_all_succeed(), so
the rewrite preserves both ordering and concurrency.

start() runs once at boot, off the i/o path, so the small overhead of
co_await over a hand-rolled continuation chain is a fine price for the
readability.

Signed-off-by: Kefu Chai <k.chai@proxmox.com>