Simplify metadata-cache.ts

[alexgleason]

Much of the code in this module is unnecessary:

• Promises can be reused. A separate data map is not needed, and is no different from just reusing the promise.
• Promise.any() can replace the firstGoodPromise() function.

[adamritter]

Hi,

sorry for not replying, just here's some feedback:

• Promise.any() had a problem of not handling errors well. I wanted to make sure that even if an error happens, if a promise fulfills later, it's returned. I should have done the error handling inside the callback maybe, but I just wanted to write that sometimes something that looks like a simplification isn't it unfortunately.
• I created the data map to be able to cache the results for a browser restart in the future. I haven't implemented it, but I think caching is important.