snapuserd: Wire up API's for Initiating and tracking Merge

Add new client API's for initiating and tracking merge.
These API's will be used by libsnapshot.

Track the merge completion in the server by walking through
all the partitions. Each worker thread will update the
merge completion as and when number of COW operations
are completed. Server will gather all the completions
of each partition and average it out. This is in sync
with the current merge completion tracking for dm-snapshot.

As a side effect, move the snapuserd_server.h/cpp files to
dm-snapshot-merge directory as it will only be a maintaining
code.

Bug: 193863443
Test: Snapuserd_test
Signed-off-by: Akilesh Kailash <akailash@google.com>
Change-Id: I031eb1a11b0f426aafbed3d39d85b0c22b9030fb