Bump AJAX_SERIAL when bumping the Extension sync generation.
Extensions often affect many aspects of page rendering. Pages that make

use of AJAX_SERIAL in their cache keys expect that a change to a

template will cause a change to the cache key. It therefore makes sense

to bump AJAX_SERIAL when we bump the extension sync generation (which

happens when enabling/disabling an extension or changing extension

configuration).
With this change, any pages that properly use AJAX_SERIAL will

invalidate their cache any time an extension makes a change that may,

for example, toggle a feature on or off.

Reinstalled Power Pack without a license. Went to a review request with

a PDF, and saw that it didn't render the thumbnail.
Set a valid license in Power Pack, reloaded the review request, and saw a thumbnail.
Disabled the extension, reloaded, and the thumbnail disappeared.
Re-enabled, reloaded, and the thumbnail appeared again.