Add memory limited bitmap pool
Default to SizedBitmapPool

Also unify builder interface re bitmap pool so
it matches memory and disk cache (either default,
an implementation, or disabled)
SizedBitmapPool -> LruBitmapPool
do bitmap ref management on a bg thread

Reduces the number of locks the main thread needs
to acquire and the amount of time they need to be
held (as well as the contention for said locks)