libsnapshot: Retrieve base sector for dm-user device creation

dm-user device requires sector information which is retrieved
from snapuserd daemon once the metadata read is completed.
Split up the initialization of daemon into two APIs. Once
the metadata read is completed, send the sector information
back to libsnapshot to create the dm-user device.

On a sidenote, remove unused code from snapuserd_client

Bug: 168311203
Test: vts_libsnapshot_test, cow_snapuserd_test

Signed-off-by: Akilesh Kailash <akailash@google.com>
Change-Id: I0647ebbeea3a4705599966974bfec2318a3e1e4e