fixes #108; uses altruist system as backup for sync check

[nymd]

Old behaviour:

• if fewer than 3 nodes in a session agreed on sync, no consensus was found, and ALL nodes were returned as viable nodes for the session

New behaviour:

• if fewer than 3 nodes in a session agree, or if any of the other possible error states happen in determining sync, the altruism system is consulted
• if altruist returns a valid blockheight, the session nodes are checked for sync against it
• if at least one node is in consensus with that blockheight, the nodes that agree are returned
• if no node agrees with the altruist, an empty set is returned which will trigger an automatic usage of the altruist for the data for that relay

Risky behaviour:

• if no nodes are found to be in sync with the altruist, the set of 0 nodes will be recorded for 30 seconds in redis as sync failures and the altruism system will take over complete service for that session for that 30 seconds