Upgrade the dcrwallet dependency to pick up the new version of
txrules.StakePoolTicketFee which considers the status of DCP0012 in its
fee calculation.
Exporting this struct is a step towards breaking up the Start func into
a New func and Run func, enabling calling code to use:
api := webapi.New()
api.Run()
WebAPI is a more suitable name than server because it matches the
package name, and also it helps to distinguish WebAPI from the HTTP
server it uses internally (ie. webapi.server rather than server.server).
