Add "warmup mode" for RPC server.

Start the RPC server before doing all the (expensive) startup initialisations like loading the block index.  Until the node is ready, return all calls immediately with a new error signalling "in warmup" with an appropriate status message (similar to the init message).

This is useful for RPC clients to know that the server is there (e. g., they don't have to start it) but not yet available.  It is used in Namecoin and Huntercoin already for some time, and there exists a UI hooked onto the RPC interface that actively uses this to its advantage.
I thought about that and decided not to do it (at least for the first proposal) because the init message is translated and the RPC status message (if it is kept generic) might be interpreted by clients in other ways than just presenting it to a user literally.  (Think:  RPC is an interface for machines not (only) humans.)  But I'm definitely open to changing that if you think it is better to reuse the init message literally.