dist_array: cache optimization for overlapped misses

[Dan Bonachea]

Copied from extras PR #12:

https://bitbucket.org/berkeleylab/upcxx-extras/pull-requests/12/wip-for-dist_array/diff#comment-137073896

second datastructure to track futures of outstanding fetches, so that overlapped ptr calls requesting the same base can elide the fetch injection and chain off the same future.

[Dan Bonachea]

This was resolved in extras PR 22, merged at d30b7c6 and to master on 5a3e7c3