crimson/os/seastore: replace Transaction block lists with PMR vectors

Replace the eight std::list block-extent members in Transaction with
std::pmr::vector backed by a 4 KiB inline arena (monotonic_buffer_resource).
Typical transactions (< ~200 entries) avoid the heap entirely.

Downstream signatures (dispatch_result_t::delayed_extents,
write_preallocated_ool_extents, update_lba_mappings, rbm_pending_ool_t)
are updated to std::vector accordingly.  The std::list<view_ref> views
member is unchanged (stable-reference requirement).

Signed-off-by: Ronen Friedman <rfriedma@redhat.com>