New volume trimmer (#113)

* Fixes for #85, #86, #94, #99, #101.

* Fix for #87, #98, #100; upgrade to batteries/0.44.1.

* wip - before major rewrite of VolumeTrimmer to simplify.

* wip - save VolumeCommittedJobTracker before removing (just in case we change our mind!)

* wip - new VolumeTrimmer.

* wip - volume trimmer fixes (plus test fixes).

* (re-)Acquire job grant first on recover in trimmer test; this is
roughly equivalent to resolving jobs first (volume.cpp)

* Added cached pool for RingBuffer, fixed testdata bug (TrieTest).

* Upgrade to batteries/0.44.1.

* Add caching of ring buffer mapped memory to slow resource growth.

* Use madvise; minimize calls to munmap.

* Fix for #108, Fix for #107.

* merge RingBuffer changes.

* Cleanup.

* fix header order

* Remove dead code.

* Only clear one mirror when deallocating a RingBuffer::Impl.

This seems to fix a sporadic Bus Error when trying to memset the
entire mapped region (size * 2) at once.

* memfd_create instead of tmpfile, normalized subpool sizes.