snapuserd: Send header-response once per entire IO request

Header response from daemon is sent once at the
beginning of the payload. This is a corner case
when the IO request can get broken into multiple chunks
if the IO is un-aligned.

Additionally, add error checks in the IO path
to validate sector information when the read
requests are merged.

Bug: 188361387
Test: Simulate an IO request to forcefully break the IO
response into multiple chunks and verify the correctness.

Signed-off-by: Akilesh Kailash <akailash@google.com>
Change-Id: I1f4fa7a79c60493f4bbed3ad49e257098b930beb
Merged-In: I1f4fa7a79c60493f4bbed3ad49e257098b930beb