Selectively use median for FallbackProvider

[ricmoo]

Certain calls in the FallbackProvider (which now checks for a quorum) need some fuzzing, since they are not guaranteed to be equal.

For example, provider.getGasPrice and a transaction receipt's confirmations can slightly differ across multiple back-ends. In these cases the median value should be used, as this minimizes the influence a single backend can have, while also providing data that is most likely to be correct.

Tagging @yuetloo for finding this.

[ricmoo]

Also, add a priority in addition to the weight; this will allow an order to be set to be loosely followed, which also gets #588 what they want. :)

By default all the priorities will be 1, which will behave identically as it does today.

[ricmoo]

This has been added to v5. Please try it out and let me know how it works for you.

Thanks! :)