Implement subscription timeout on the server side in HTTP triggering if request entity was not read in time

**Issue by [jrudolph](https://github.com/jrudolph)**
_Thursday Aug 20, 2015 at 07:30 GMT_
_Originally opened as https://github.com/akka/akka/issues/18274_

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See http://stackoverflow.com/questions/32085345/how-to-configure-akka-http-to-return-http-503-in-case-of-request-timeouts

Not sure which publisher timed out in that specific case but if we are able to return a 500 response we should be able to deliver a 503 as well.

/cc @sirthias

The issue names no source files, tests, or entry points. Trace the server-side HTTP request-entity timeout handling first, using the linked Stack Overflow report for context; done means a request whose entity is not read in time receives an HTTP 503 response.

Written by the indexing model from the issue text.