Upgrades for extensions which include static media (like javascript or CSS) are

highly annoying, because in addition to installing the package, the user has to

disable the extension, then re-enable it. This change fixes all that.
When we install extension static media, we add a key to the extension settings

dictionary called _extension_media_version. Then, when initializing the

extension class, if the version stored in settings doesn't match the one for

the running code, we'll re-install the media.
This is fine and dandy for the single-threaded devserver, but it's possible

that installing a new extension could cause a bunch of threads/processes to try

to reinstall static media all at once on a production server. I've therefore

created a relatively simple locking mechanism to ensure that only one thread

will be installing media for any given extension at a time.
Testing done:

Enabled a new extension and saw the media get installed.
Disabled an existing extension and re-enabled it, and saw media get installed.
Restarted my devserver and saw that static media versions were checked, but no re-installation occurred.
Updated the version of an extension, restarted the devserver, and saw that static media got upgraded.
Created a lock contention and saw that initializing the extension waited for the lock to clear.