libsnapshot: Flush data to scratch space only for overlapping regions

When read-ahead thread caches the data from base device, flush the data
only if there are overlapping regions. If there is crash, subsequent
reboot will not recover the data from scratch space. Rather, data
will be re-constructed from base device.

Additionally, allow batch merge of blocks by the kernel even for
overlapping region given that we have the read-ahead thread
taking care of the overlapping blocks.

Bug: 183863613
Test: 1: Incremental OTA from build 7284758 to 7288239. Merge time
         reduces from ~6 minutes to ~2.5 minutes
      2: Reboot and crash kernel multiple times when merge was in
         progress
      3: Verify read-ahead thread re-constructs the data for overlapping
         region.
Signed-off-by: Akilesh Kailash <akailash@google.com>
Change-Id: I50e0d828f4fb36a23f0ca13b07a73229ba68874d