DeleteStaleTemplateCaches job getting stuck
See these for help debugging stuck tasks:

Generating pending image transforms stuck in process

Deleting Stale Template Caches Failed

http://buildwithcraft.com/help/stuck-tasks

As far as what cache gets saved where, template caches (the {% cache %} tag) always get saved in the database.

See here for an explanation on how all of that works.

The cacheMethod config setting, which is what you've set to memcache affects any PHP code in Craft that is using craft()->cache (including plugins).

See these for more discussion on it:

How much data can craft()-&gt;cache hold?

Use cache instead of saving to database