crimson/os/seastore: move the root meta out of the root block

crimson/os/seastore: move the root meta out of the root block

During massive data backfilling, new osdmaps keep being created due to
frequent pg status changes, which can lead to frequent osd meta updates.
Those updates will be translated into "SeaStore::write_meta"s, which
modifies the root block's meta field and invalidates all inflight
transactions. Since the osd meta updates can be very frequent, long
transactions may be kept invalidated and the corresponding IO requests
hang.

This commit moves the root meta out of the root block, so that updates
to it won't invalidate irrelevant transactions

Signed-off-by: Xuehan Xu <xuxuehan@qianxin.com>