Fix extent_grow_next management.

Fix management of extent_grow_next to serialize operations that may grow
retained memory.  This assures that the sizes of the newly allocated
extents correspond to the size classes in the intended growth sequence.

Fix management of extent_grow_next to skip size classes if a request is
too large to be satisfied by the next size in the growth sequence.  This
avoids the potential for an arbitrary number of requests to bypass
triggering extent_grow_next increases.

This resolves #858.