Don't challenge without GPS lock close to assert

Add audit 27, snapshots

ignore if we can't take a snapshot

snaps try 3

ledger snapshots

Allow for the taking and loading of ledger snapshots. Once a chain var
has been activated, consensus members will periodically publish snapshot
hashes to the chain that can be used for syncing and detection of ledger
drift.

Add audit 26

Add audit 25

Fix tests and update core

update assumed valid and blockchain dep