storage: fix the max supported key and key part length
storage: fix the max supported key and key part length

It doesn't change any behavior because handler class has
MAX_KEY_LENGTH (= 3072) check.

Groonga can handle 4096 byte for a key but we can't use it on Mroonga
because of MySQL limitation. :<

mroonga/mroonga,naoa/mroonga,yoku0825/mroonga,yoku0825/mroonga,yoku0825/mroonga,naoa/mroonga,mroonga/mroonga,mroonga/mroonga,naoa/mroonga,yoku0825/mroonga,yoku0825/mroonga,naoa/mroonga,yoku0825/mroonga,naoa/mroonga,naoa/mroonga,mroonga/mroonga