Add support for using multiple Base Nodes in the wallet

[philipr-za]

Currently the wallet only supports having a single Base Node assigned to it for all its blockchain monitoring and transaction broadcast needs. If the base node is down then the Wallet cannot check the blockchain to make sure its outputs are still spendable or to monitor the status of broadcast transactions.

It would be more robust if a set of Base Nodes can be added to the Wallet. The Liveness of these nodes can be monitored via the Liveness service and then all these nodes can be used for the following functionality:

• Confirm with multiple BN's whether the unspent outputs are still unspent or should be invalidated.
• Broadcast transactions to mutliple base nodes
• Monitor transaction status on multiple base nodes.

[philipr-za]

After discussion with the Mobile team the public keys of the base nodes will not be persisted by the backend and will be provided by the client on startup. The backend will provide two functions:

1. Append new BN public key to the list
2. Clear the list completely.

[philipr-za]

The service that provides this functionality will also monitor the current chain tip from the nodes as described in #1806