Add functionality for Wallet to monitor the current chain tip and sync status from its base nodes

[philipr-za]

Currently the wallet has no knowledge of the current chain tip in the network. It would be useful to have this information for the following reasons:

• Knowing which outputs have matured or when they will mature for inclusion in transactions
• Being able to label a transaction output with a maturity to indicate its age.
• Being able to know if a base node has synced before checking with it if our outputs are spendable.

In order to monitor the current chain tip the Wallet will need to enquire the current chain status from its base node. This is done in the Liveness service currently and should be made available to the Wallet.

Effort must be made to determine of the Base Node is currently synced with the rest of the network which might involve requesting this info from multiple BNs using the functionality from #1805

[philipr-za]

#2194 provides the sync status.

It will still be useful to add a Base Node monitoring service to the wallet that maintains knowledge of the current chain tip.