Restructure hash calculation in delta_performer

This patch moves DeltaPerformer::CalculateAndValidateHash to
fd_utils::ReadAndHashExtents and cleans up the code. It also adds
unittests for ReadAndHashExtents.

Bug: None
Test: unittest pass

Change-Id: I297cf79ef38a7495d5bcc0e6516a1ca783e505ea
Signed-off-by: Amin Hassani <ahassani@google.com>