Rename Dao to DaoSpi, add caching Dao
Rename Dao to DaoSpi, add caching Dao

Let the new Dao delegate database requests to a DaoSpi implementation,
and cache pages in a ConcurrentMap.

debiki/debiki-server-old,debiki/debiki-server-old,debiki/debiki-server-old,debiki/debiki-server-old