msg/async/frames_v2: rename and clarify FRAME_FLAGS_LATEABRT

msg/async/frames_v2: rename and clarify FRAME_FLAGS_LATEABRT

Clarify that the frame can be aborted at any point after the
preamble and the first segment are put on the wire.  When that
happens, the remaining segments (including the data segment)
may be filled with zeros.

Signed-off-by: Ilya Dryomov <idryomov@gmail.com>
(cherry picked from commit bc835b8bed627af89b1a0bb0f7585e0e3d531aaa)

Conflicts:
	src/crimson/net/ProtocolV2.cc [ crimson doesn't support
	  msgr2 in nautilus ]
	src/msg/async/frames_v2.h [ context: commit c70f779d12a2
	  ("headers: Make ceph_le member private") not in nautilus ]