filter caching (done :)

Hi people!

After we have turned on bbcode, glossary, html escaping, and some custom 
developed filters (including a small bbcode addon and an email address 
obfuscator) we have experienced our Drupal site producing pages very 
slowly. We have checked memory, our opcode caching, but everything 
seemed to work fine, so it seemed that the filters are slowing 
everything down.

So I have digged up the filter caching patches from Steven and adapted 
them to current CVS needs (dropping the $donotcache stuff on the way, 
especially that I needed a semi-drop-in solution as much as possible). 
Here it is: a filtercache.module with its own database table which plugs 
into Drupal to cache the filtered texts, thus speeding up sites with 
expensive filters a lot.

It needs some small core changes, so check_output() is not defined in 
filter.module if filtercache.module is present and the cache is emptyed 
if any filter settings change, or modules get enabled / disabled.

We have apachebenched our site without filter caching and with filter 
caching, and we experienced an average of five times speed increase if 
caching is used. Therefore I would only suggest it to anybody who is in 
the trouble of not being able to use expensive filters because of their 
extremely slow code... Feel free to do more benchmarks against it :)

Steven: this renders the filter caching stuff in your sandbox obsolote, 
but I have not removed it, but let you do it...

Goba