This change introduces additional network activity with the btcd
process to ensure that the network connection is not silently dropped.
Previously, if the connection was lost (e.g. wallet runs on a laptop
and connects to remote btcd, and the laptop is suspended/resumed) the
lost connection would not be detectable since all normal RPC activity
(excluding requests from btcwallet to btcd made by the user) is in the
direction of btcd to wallet in the form of websocket notifications.
This a refactor of the btcwallet main package to create a new wallet
package.
The main feature of this package is the integration of all the other
wallet components (waddrmgr, txstore, and chain) and the Wallet type is
'runnable', so it will be continuously updating itself against changes
notified by the remote btcd instance.
It also includes several methods which provide access to information
necessary to run a wallet RPC server.
