lcb_create: Initialize 'instance' field for cmd

This is a bit of a hack as the cmdq shouldn't have anything to do with
the instance, nevertheless, some things still use it.

Change-Id: I6672550127d87a3436a9e30b02cbac2d1a8d300f
instance.c assignment fixup

Change-Id: I77a423888764a8a2fe584991fd3d1209321d0943
Send keys with -1 index to a random server.

The aim here is that the server will repy with a NOT-MY-VBUCKET using an
updated configuration. At this point it is better than returning an
error immediately to the user, since in that case the client never has a
chance to refresh the config.

Change-Id: I50e7db48f67f694c97c5b10b7c64bceed9da4d96