await pending future

[makeroo]

It's a possible solution for https://github.com/hephex/asyncache/issues/2

When invoking the same cached func multiple times, if the future returned by the first call is still pending then wait for it instead of triggering a new call.

Note: do not cache errors. When the call fails, the future is removed from the cache.

Note 2: this behaviours should be customizable (ie. error handling, multiple calls, etc.)