[cliquet] add a Server-Flush header

original: https://github.com/mozilla-services/cliquet/issues/536

@leplatrem suggested:
- The `/__flush__` endpoint stores the current timestamp (current time) in the cache backend
- We add a header Server-Flush that gives the value to the client
- With memory backend, we set Server-Flush on server start
- The client obtains this server-flush, if it is superior to _since then it means the server was flushed meanwhile

@natim said he liked the solution

Start at the /__flush__ endpoint and trace how the cache backend stores the current timestamp, including the memory backend's server-start behavior. Check how clients compare Server-Flush with _since; the work is done when the header is exposed consistently and the flush-detection behavior matches the described flow.

Written by the indexing model from the issue text.