worker: remove freelists for iohandle and iorequest

worker: remove freelists for iohandle and iorequest

A quick profiling showed no change in performance,
and in particular no change in time spent in malloc/free.
Some of the types in the union differed in size by a multiple.

If their performance won't be satisfying, replacements should be
considered first (e.g. jemalloc) before rolling our own stuff.