[Serve] Added deadline awareness (#6442)

* [Serve] Added deadline awareness

Added deadline awareness while enqueuing a query
Using Blist sorted-list implementation (ascending order) to get queries according to their specified deadlines. [buffer_queues]
Exposed slo_ms via handle/http request
Added slo example 
The queries in example will be executed in almost the opposite order of which they are fired
Added slo pytest
Added check for slo_ms to not be negative
Included the changes suggested

* Linting Corrections

* Adding the code changes suggested by format.sh

* Added the suggested changes

Added justification for blist
Added blist in travis/ci/install-dependencies.sh

* Fixed linting issues

* Added blist to ray/doc/requirements-doc.txt

python/setup.py

@@ -79,7 +79,7 @@ extras = {
     ],
     "debug": ["psutil", "setproctitle", "py-spy >= 0.2.0"],
     "dashboard": ["aiohttp", "google", "grpcio", "psutil", "setproctitle"],
-    "serve": ["uvicorn", "pygments", "werkzeug", "flask", "pandas"],
+    "serve": ["uvicorn", "pygments", "werkzeug", "flask", "pandas", "blist"],
     "tune": ["tabulate"],
 }