Westlad/proposer robustify

[Westlad]

fixes #598

This PR adds some additional robustness to websockets:

1. proposer, challenger and liquidity provider all ping the websocket server (optimist) to help prevent the websocket closing due to idleness. They log any timeouts.
2. optimist pings all connected websocket clients. If it doesn't receive a pong in time, it will terminate the websocket...
3. ...this will cause the client to undergo a re-connect cycle to attempt to re-establish a new connection.
4. optimist will stop sending blocks for signature (it will go into a wait loop) until a new connection is established.

The web3js websocket cannot be handled exactly in this way because it's used indirectly via web3js, but a ping is made every 15 seconds by querying the latest blocknumber, and the connection is already tested every 2s and a reconnect attempt made if it is closed.

Testing can be done by running the ping-pong test (no relation). During the test use docker-compose pause proposer to stop the proposer. After about 30s optimist will notice that the websocket is closed and go into a wait loop (indicated in the logs) waiting for proposer to reconnect. Unpausing the container (docker compose unpause proposer) will allow proposer to realise the connection has dropped and it will re-establish the websocket, at which point optimist will resume normal operation.

Do not, however, expect the test to continue with no errors. It takes time for an application to realise the websocket has dropped, and during that time the state changes are not properly communicated to the other containers. That's ok - this mechanism is really meant to work when a connection is idle, not when it's running a test. If a websocket drops during a period of activity, it would normally mean there is a problem with a container. That's not in the scope of this PR.

Note: this PR also fixes a small bug that can cause block leafCounts to be computed incorrectly if several blocks are made quickly (local leafCounts in the Block class did not increment correctly).