Support for read-only operations on replicas

[xiam]

Some databases support replication features. upper.io/db should distinguish between write operations (INSERT, UPDATE, DELETE) and direct them to master, read-only operations (SELECT) and direct them to slave[1], slave[2], ..., slave[n].

[xiam]

There's no need to do this. Users should be able to design their own environment and to decide how do they want to use it.