libsnapshot:snapuserd: read-ahead COW copy ops

Introduce read-ahead mechanism for COW copy ops.

1: Read-ahead thread will read from base device
   and store the data in scratch space along with the metadata.
2: Worker threads during merge will retrieve the data
   from read-ahead cache
3: Fixed set of blocks are read during each cycle by the read-ahead
   thread.
4: When the last block in the region is merged, read-ahead thread
   makes forward progress.

Scratch space is set to 2MB and is only used from COW copy operations.
We can extend this to Replace Ops based on performance evaluation.

Performance:

As mentioned in bug 181883791, Incremental OTA of size 55M with
235K copy operations where every block is moved by 4k:

Without read-ahead: 40 Minutes for merge completion
With read-ahead:  21 Minutes for merge completion

Bug: 183863613

Test: 1: Full OTA - no regression observed.
2: Incremental OTA - with older COW format. Daemon will just skip
   the read-ahead feature for older COW format.
3: Incremental OTA - with new COW format.
4: Reboot and crash kernel when multiple times when incremental OTA is in-flight.
   Verify post reboot, read-ahead thread re-constructs the data from scratch
   space.
5: No regression observed in RSS-Anon memory usage when merge in-flight.

Signed-off-by: Akilesh Kailash <akailash@google.com>
Change-Id: Ic565bfbee3e9fcfc94af694596dbf44c0877639f