Get status of in progress transactions

[cmeisl]

Although client can reconnect to server websocket and pick up where it left off with in progress notifications, only new notifications are shown.

Provide a new API to get the current state of all in progress notifications. This will allow the client to make decisions on tracked transactions when the user may not have seen the notification, e.g. of connection was lost when transaction was confirmed.

Only transactions known by Assist's transaction pool (i.e. sent but not yet confirmed) would be supported by this feature.

[cmeisl]

This will require additions to the backed to support the querying state of transactions.