We think we have memory fragmentation causing failed extraction in
Windows containers and smaller Unix devices.

Writes of both full objects and streamed objects now re-use the Vec via
a sharded-slab implementation. To facilitate the more complicated memory
logic, buffer limit management is now integrated into the IO Executor:
the immediate executor doesn't limit at all as no outstanding buffers
occur, and the threaded executor tracks both the total allocated buffers
as well as whether a reusable buffer is available.