Consider routing requests transparently to leader node if necessary

[otoolep]

In principle it's possible to have a node re-route a HTTP request to the leader node, if a follower receives it. Right now that is not the chosen behavior, and the follower node returns with HTTP 301. This provides maximum transparency to clients, allowing them to talk directly to the leader and be sure that a command has been accepted.

[otoolep]

A note about the current design has been added here: https://github.com/rqlite/rqlite/blob/master/doc/DATA_API.md#sending-requests-to-followers

It should be noted that adding this feature introduces a lot of possible error cases, that need to be handled with timeouts. Not ruling it out, but it must be considered carefully before allowing it. At the very least, it must able to be disabled by the client if it chooses.

[otoolep]

Will be fixed by #859