Always use the DB name as the cache key

mango_idx:list/1 accepts a #db{} in addition to a database name. When
the former is used it's often a stub record with a randomly generated
UUID in the #db_header. This really kills the cache hit rate ;)

BugzID: 42707
Merge pull request #16 from cloudant/42707-always-use-dbname-cache-key

Always use the DB name as the cache key