I am closing this as abandoned. If it gets reopened things to look at:

  1) support anydbm
  2) consider making it support redis instead/in addition to memcached
     since redis is supported for sessiondb.
  3) add a [dbcache] config section for config options.
     (similar to how [sessiondb] was added with redis support). Might need a time
     to live setting or max-size setting to imit cache size.
  4) document which pip install command is used to supply the 'memcache' module.
  5) this patch cascades caches. It calls memcached on a cache miss of the in memory
     cache, then goes to the DB. Not sure how two roundup processes
     behave if: roundup1 caches X, roundup 2 updates X. Roundup2 can evict X from
     in memory and memcached, but how is X evicted from roundup1? I think the cascade
     needs to be removed and only memcached used as the cache.