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).
It's generally good practice to first create everything that can fail
and then create the final instance at once with the results versus doing
it piecemeal.
Piecemeal creation is typically more error prone and, while not a huge
concern here, it also ends up needlessly creating objects that are just
thrown away in the event of a later error.
Storing references to the database/dcrd/dcrwallet clients inside the
cache struct means they don't need to be passed in every time the cache
is updated.
