masternode: fix dseep DoS

When a dseep message with an invalid signature is received, the
lastDseep timestamp is updated to sigTime, but the message rejected.
Further messages with a sigTime before lastDseep will not be
processed, meaning the LastSeen timestamp will not get updated.

By disabling dseep processing for more than 65 minutes an attacker
can cause a master node to time out.

Only update the lastDseep stamp if the message is signed with the
correct key.

Signed-off-by: Patrick McHardy <kaber@trash.net>