rtio: Add a context pool

Adds a context pool that can be used when a number of threads may be
dynamically created to use with RTIO. While the pool has a small cost to
it, the cost of verifying a statically declared kobject likely makes it
worth avoiding dynamically allocating these with the typical
kobject_alloc method. Instead this arena style pool of objects can be
used where the kobject validation uses the gperf hash created at build
time.

Signed-off-by: Tom Burdick <thomas.burdick@intel.com>