Change index creation time from being on connect to on create

[Kezzsim]

Users at NSLS-II have reported that connections handled through suitcase mongo can be exceptionally slow when searching for data. Part of this might be from superfluous use of pymongo.serializer in certain methods like update_metadata, but we've identified that another cause is the client's attempt to create indexes upon each connection.

This PR is an attempt to address #37

Tasks required

• Make create_index a public method
• Remove that method from _init_ so it does not run every time
• Update docs to inform users that they need to create indexes manually when setting up or migrating a mongo tiled database.
• Pass all tests