Consensus ledger switch improvements

* Expire validations faster based on when we first saw them.
* Never jump to a ledger prior to the latest fully-valid ledger
* Drop validations with signing times too far in the future immediately

src/ripple/app/ledger/impl/LedgerMaster.cpp

@@ -198,7 +198,7 @@ public:
         if (mStrictValCount)
         {
             // If we're only using validation count, then we can't
-            // reject a ledger even if it's ioncompatible
+            // reject a ledger even if it's incompatible
             return true;
         }