[Cashtab] Use promise.all to debottleneck NFT offer loadtime

Summary:
Initial release of NFT trading was not written to account for large numbers of offers and tokenIds. Lots of debottlenecking opportunities.

Big one is using promise.all to get and set agora offers.

Implement this.

Since we also want to implement promise.all to get and cache token info, and the approach is similar -- we do this in the same diff. In both cases, testing is the same, and more improvements are available.

imo worth doing this in one diff to solve the immediate problem of "this is almost unusable at current load speeds." We can optimize from here.

Test Plan:
npm test

npm start and open localhost:3000 in an incognito window. Create a wallet and navigate to the Listed NFTs screen. Note that the UI loads reasonably quick (with some spinners going for token names).

Reviewers: #bitcoin_abc, emack

Reviewed By: #bitcoin_abc, emack

Differential Revision: https://reviews.bitcoinabc.org/D16762