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.
