WL#1739: Handler: use HA_EXTRA_CACHE

HA_EXTRA_CACHE should be used with position() and rnd_pos() to cache the 
records marked tith position(). They can then be fetched directly from the 
cache wqhen rnd_pos is called. The size of this cache is received with extra_opt
(HA_EXTRA_CACHE, size_of_cache).
Make class for this cache, where size_of_cache is in bytes.