libsnapshot:snapuserd: mmap + msync header after merge

mmap the CowHeader and use msync to flush only the
first 4k page after merge is complete.

This cuts down ~30 seconds of merge completion time
on a 55M incremental OTA with 235k copy operations.

Although, this isn't a significant gain but this patch
creates a scaffolding for the next set of read-ahead patches.

Bug: 183863613
Test: Incremental and Full OTA
Signed-off-by: Akilesh Kailash <akailash@google.com>
Change-Id: I15bfec91ea1d5bdf4390670bcf406e1015b79299