When one wants to browse a revisions log using the browse web application in production, an error is raised when going back far in time (see http://localhost:5004/browse/origin/https://github.com/ocaml/ocaml/log/?offset=1600&per_page=100&branch=refs/heads/3.08 for instance).
The reported error is the following:
```
[06/Jun/2019 09:56:03] [ERROR] <WSGIRequest: GET '/browse/origin/https://github.com/ocaml/ocaml/log/?offset=1600&per_page=100&branch=refs/heads/3.08'> 500
Traceback (most recent call last):
File "/usr/lib/python3/dist-packages/django/core/handlers/exception.py", line 41, in inner
response = get_response(request)
File "/usr/lib/python3/dist-packages/django/utils/deprecation.py", line 142, in __call__
response = self.process_response(request, response)
File "/usr/lib/python3/dist-packages/django/contrib/sessions/middleware.py", line 58, in process_response
request.session.save()
File "/usr/lib/python3/dist-packages/django/contrib/sessions/backends/cache.py", line 57, in save
return self.create()
File "/usr/lib/python3/dist-packages/django/contrib/sessions/backends/cache.py", line 52, in create
"Unable to create a new session key. "
RuntimeError: Unable to create a new session key. It is likely that the cache is unavailable.
```
The error is likely due to the fact that the maximum size for a memcached cache entry is reached.
Indeed, we use a cache here to backup the revisions ordering computed so far and allow pagination.
This needs to be fixed.