Heartbeat validator message

[Ivshti]

Periodically sign a Heartbeat event and send it to the other validators; this event might contain some data to distinguish the time, e.g. timestamp or sequence

this would solve #13, help with #6 and help the market determine the health of the channel

more accurately, it allows the market to distinguish between more detailed off-chain state: a waiting state where we are waiting for all validators to post a heartbeat, and then offline (a type of unhealthy state) which happens if some validator hasn't sent a heartbeat recently;

once this is done, tests must filter validator messages by type (otherwise the last messages will be full of Heartbeat)

see also: http://github.com/adexnetwork/adex-market/issues/1 and https://github.com/AdExNetwork/adex-protocol/blob/master/components/market.md