use lock.inc instead of a file lock

your patch:

1. adds @ on getimagesize to suppress warning messages
2. adjusts relevant comments in code to reflect change in lock system
3. changes lock system to use a database lock

It still probably needs to

1. lock_release after creating the image to prevent default 30 second timeout on parallel request
2. send separate watchdog messages for the case of "received lock but failed to generate images" and "saw a lock in place but after the lock expired there was no derivative image"

I might reword the second error message for clarity to say something like "...but found no generated image from %image using imagecache preset %preset."

After that I will run the patch and hopefully a couple of other people will try it so it can be ready for whenever the next commit comes.

Looks like this works fine.

I don't have a way of generating a series of racing request which are certain to collide in order to test every option, but this does fix the infinite loop problems and holds multiple requests until the image generates.

Marking as ImageCache 3.x Todo.