Perform a consistency check before deleting snapshots.

If for some reason the COW state is not fully synced to disk, but
dm-snapshot has flushed its pending merges, we do not want to delete
snapshots. Doing so could potentially leave blocks unmerged.

This situation is quite unexpected so we label it as a merge failure.
The device can recover by completely syncing the COW state, and then
rebooting, which will attempt to make forward progress on the merge.

Bug: 190582627
Test: vts_libsnapshot_test
      full OTA on bramble
      incremental OTA on bramble
Change-Id: Ib887f1d9e4397a712ed2f800cc1222cf9305a039
Merged-In: Ib887f1d9e4397a712ed2f800cc1222cf9305a039