Allow sending commands to all shards

For some use-cases, being able to send a CREATE DATABASE or other CREATE statement to all nodes would be useful. Examples of this are creation of commonly shared data tables.

A possible implementation of this would be to implement handling of the ROUTE_TO_ALL hint in the schemarouter. This is currently an unused hint but using it for this purpose would seem useful.

For commands that return results, an unordered "union" of the results could be created. This may be useful for collecting system information from all nodes while also allowing things like integrity checks with CHECKSUM TABLE.