Add OptimizeSourceCopyOperation

... so that an operation can be skipped partially. For example, if
an operation contains blocks:
    563412 -> 123456
... then optimized operation is:
    5612 -> 1256

Test: update_engine_unittests
Test: apply incremental OTA
Bug: 148623880

In an experiment, this reduces CoW size of an incremental update
package by 200MB (out of 700MB).

Change-Id: I86ca23fd589ddbc84c81318283b5f4e71782a759