snapuserd: Implement snapshot merge

Implement snapshot merge in the daemon. Following
are the important changes:

1: Spin up merge thread which does merging in user-space.
2: For ordered ops (COPY and XOR), read-ahead thread is used.
3: Read-ahead thread will read a fixed set of COW ops and cache
them in memory. Furthermore data is saved in scratch space buffer
in COW device.
4: No change in scratch space buffer - default 2MB buffer is allocated.
5: Merge thread and Read-ahead thread will work in lock step for merging
ordered ops.
6: Once the ordered ops are merged, REPLACE and XOR operations are
merged.
7: If there is a crash during merge, COW header tracks the number of
operations merged. No change in this code path.
8: Merge thread requires Base device for merging as opposed to using
the source device. Hence, while initializng the merge threads,
libsnapshot will have to pass the "Base" device to the daemon. This
is the same "Base" device which is passed to dm-snapshot during
snapshot creation.

Patch does not handle any communication with dm-user yet.

Bug: 193863397
Bug: 193863280
Bug: 193862712
Test: snapuserd_test on CF
Signed-off-by: Akilesh Kailash <akailash@google.com>
Change-Id: I14aab6eaa07ac68f2a3a23516ed9ba6567a35734