Separate arena_avail trees

Separate run trees by index, replacing the previous quantize logic.
Quantization by index is now performed only on insertion / removal from
the tree, and not on node comparison, saving some cpu.  This also means
we don't have to dereference the miscelm* pointers, saving half of the
memory loads from miscelms/mapbits that have fallen out of cache.  A
linear scan of the indicies appears to be fast enough.

The only cost of this is an extra tree array in each arena.