#4039 Allura should be able use mongo slaves
Right now all reads come from the mongo master, and we don't take advantage of the slaves. This is good because it means we also have current data when displaying a page (slaves can lag behind) but it also means we're not very efficient. Distributing reads to a slave would be helpful.
Ideas:
- use 'w' option to ensure writes propagate to slaves
- that will slow down the writes, but we should make sure we don't have unnecessary writes anyway
- http://www.mongodb.org/display/DOCS/getLastError+Command
- http://api.mongodb.org/python/current/api/pymongo/collection.html#pymongo.collection.Collection.set_lasterror_options
- add app logic to figure out when a session has made changes (logged in, some anonymous sessions) and use the master for reads, so data is current. All other reads can go to slaves.