Merge branch 'ps/reftable-iterator-reuse' into jch

Optimize reading random references out of the reftable backend by
allowing reuse of iterator objects.

* ps/reftable-iterator-reuse:
  refs/reftable: reuse iterators when reading refs
  reftable/merged: drain priority queue on reseek
  reftable/stack: add mechanism to notify callers on reload
  refs/reftable: refactor reflog expiry to use reftable backend
  refs/reftable: refactor reading symbolic refs to use reftable backend
  refs/reftable: read references via `struct reftable_backend`
  refs/reftable: handle reloading stacks in the reftable backend
  refs/reftable: encapsulate reftable stack

Author: gitster