Convert SnapshotStatus to proto

Also, add a "name" field to SnapshotStatus, and delete
the "name" arg from CreateSnapshot / WriteSnapshotStatus.
ReadSnapshotStatus will warn if the name mismatches from
the file name, and auto-correct it.

Test: libsnapshot_test

Change-Id: I725cf39c07684b100b140a8a21ea9d23ab9d2241
9 files changed