BEP Idea: Follower nodes

Add docs, tooling and code to help set up "follower nodes" (or "user nodes"), i.e. nodes which stay in sync with a BigchainDB network, but which can't vote or post new transactions. End users could query them, for example, but they would reject all POST requests (with new transactions). They could have specialized MongoDB indexes for particular use cases.

No files, tests, or entry points are named. Start by reviewing the proposal's requirements for follower or user nodes, including synchronization, query access, rejected transaction POSTs, and specialized MongoDB indexes; done means the requested documentation, tooling, and code are all scoped and implemented.

Written by the indexing model from the issue text.