Change Opensea update style

[JamesSmartCell]

To prevent hammering the opensea API change the update system to:

1. Call opensea once after user opens wallet or changes network. No subsequent calls.
2. Maintain sync by using event logs or transfer logs (where supported) from here on. Note this mechanism is already in place.

[JamesSmartCell]

Update: It seems OpenSea is very slow to adapt to self-destructed tokens. It will still return the presence of destroyed tokens way after they've gone. This results in the token destruction detecting mechanism in AW fighting with OpenSea.

Because self-destruct is going out of fashion this may not be much of an issue.