sync doesn't cancel when creating new wallet

[dreacot]

It was noticed on godcr that while syncing a wallet, and attempting to create a new wallet, the log output gets stuck on [INF] DLWL: Canceling sync. May take a while for sync to fully cancel., it's important to know the UI is still usable when this happens, quitting the app and reopening shows that the new wallet was never created.

The error was easily reproduced on a fresh install of godcr with no prior databases.

This dcrlibwallet PR planetdecred/dcrlibwallet#156 added the capability for the library to automatically cancel sync while creating/deleting a wallet, as opposed to doing that manually in the client apps.

The change was implemented into dcrandroid by this PR planetdecred/dcrandroid#534, and the functionality was tested to work as intended.

The change was implemented into dcrios by this PR https://github.com/planetdecred/dcrios/pull/736/files, and the functionality was tested to work as intended.