Feature/coalesce handler

[ajuvercr]

When a requests hits the cache, the cache might have an instant cached response.
When the cache gets 2 requests simultaneously with the same key, so the second should get a cached request, yet it doesn't because the cache is only updated after the request is finished.

What happens:

          cache
req1   \   |
         \ |
req2   \   |
         \ |
         / |
req1   /   |
         / |
req2   /   |

Expected:

What happens:

          cache
req1   \   |
         \ |
req2   \   |
         \ |
         / |
req1,2 /   |

This pull request alleviates this by creating a Coalescer during the request on which incoming requests can attach and get the same response as the starting request.