Add xent_collection_admin

[yuema137]

In the past, we used secondaryPreferred in the pymongo interface for all purposes, which was not reasonable. A better practice is to:

• Use secondary only for normal read-only operations to avoid crushing the primary when the connection demand is high
• When writing to the database, we should use xent_collection_admin, which only connects to the primary. Connection to secondaries is unnecessary here as there is no writing option for those.