Fix multiple snapshots present after journal removal

After journal removal the sequence number starts counting from 0
once again. Therefore we can have multiple snapshots present and
snapshots with higher sequence number always take priority for loading
even when a newer snapshot is present.

Change this up in 2 ways:
1. disregard sequence number while deleting snapshots(delete all
snapshots with an older timestamp)
2. while loading snapshots prioritize timestamp rather than seqNr.

Also simplify LocalSnapshotStore.compare().

Change-Id: I205ea0ddf48d73b0a09297a1ce4e9fd514531993
Signed-off-by: Tomas Cere <tomas.cere@pantheon.tech>
Signed-off-by: Robert Varga <robert.varga@pantheon.tech>