New bigchaindb Command to Shut Down BigchainDB Server Cleanly

We need a way to shut down BigchainDB Server cleanly and gracefully (e.g. a new bigchaindb something command-line command):

• even if new HTTP requests continue to arrive at port 9984 (and are expected to keep arriving)
• even if the node is part of a live cluster
• even if new RethinkDB traffic (e.g. new transactions in the backlog table, etc.) is arriving on port 29015

We might allow RethinkDB to keep running. For example, if we want to change a BigchainDB Server configuration setting (e.g. the keyring), then all we really need to do is shut down BigchainDB Server. RethinkDB can keep running.

A sketch of what the command might do:

1. Stop accepting new requests via the HTTP API (on port 9984)
2. Block incoming traffic on port 29015 (but RethinkDB can be kept running)
3. Wait for all BigchainDB queues (in multipipes) to become empty
4. Stop all BigchainDB Server processes gracefully

This issue was inspired, in part, by issues #357 and #436

@vrde I'm going to assign this to you since I think you know the most about how to do it, being very familiar with the bigchaindb command and also multipipes.