mtd: blktrans: remove mtd_blkcore_priv, switch to per device queue and thread

This is the biggest change. To make hotplug possible, and this layer
clean, the mtd_blktrans_dev now contains everything for a single mtd
block translation device. Also removed some very old leftovers.

Signed-off-by: Maxim Levitsky <maximlevitsky@gmail.com>
Signed-off-by: David Woodhouse <David.Woodhouse@intel.com>