View groups keep their databases open all the time

As recently discussed/reported in the users mailing list, view groups keep databases open all the time (without the need to) which is a problem once max_dbs_open is reached.

The relevant thread is the following:

http://mail-archives.apache.org/mod_mbox/couchdb-user/201104.mbox/%3CBANLkTi=vUR4uadBLW49HEcQarB8UJ2TWrA@mail.gmail.com%3E

The following patch, with an etap test, fixes it. It's also a simpler solution for COUCHDB-926.