Horizon filtering not thread-safe

[Erik Olof Gunnar Andersson]

The Horizon filtering is not fully thread-safe when two users are simultaneously filtering a resource (e.g. projects or instances) and threading is enabled (e.g. 2 or more threads in Apache). It's possible that another user's filter will be displayed in the form input.

I was able to reproduce the bug using devstack; I logged in simultaneously with different accounts using two different browsers. Then, I typed different filter terms on each account and after spamming the filter button on both, the filter term from one was eventually transferred over to the other.

It's easier to reproduce if configured with only a single process.
> WSGIDaemonProcess horizon user=stack group=stack processes=1 threads=10 home=/opt/stack/horizon display-name=%{GROUP}