Skip stashing source blocks in verify mode

Currently block_image_verify() stashes source blocks to /cache and
in some case triggers I/O errors. To avoid this risk, We create
a map from the hash value to the source blocks' range_set. When
executing stash command in verify mode, source range is saved but block
contents aren't stashed. And load_stash could get its value from
either the stashed file from the previous update, or the contents on
the source partition specified by the saved range.

Bug: 27584487
Bug: 25633753
Change-Id: I775baf4bee55762b6e7b204f8294afc597afd996