snapuserd: Service I/O requests from dm-user

Now that merging is done in user-space and
partitions are mounted off dm-user directly,
daemon will have to serve every I/O request.

Daemon has to handle this wherein we need to
check if the given I/O request block has been
modified in the OTA. Furthermore, if merge is
in-progress, we will have to synchronize with
the merge thread before servicing the I/O.

If the I/O request maps to a REPLACE or ZERO op,
we will just read the data from COW device.

If the I/O request maps to a COPY or XOR op,
the worker thread will have to synchronize
with the merge thread and if the merge is
in progress, fetch the data directly from RA buffer.

This patch handles I/O requests only if the
sectors are 4k aligned.

Bug: 196929997
Test: snapuserd_test

Signed-off-by: Akilesh Kailash <akailash@google.com>
Change-Id: I08562b8927e1c22dd9d9ef160e873280854eac99