Merge branch 'ps/reftable-iterator-reuse'

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: figure out hash via `reftable_stack` reftable/stack: add accessor for the hash ID refs/reftable: handle reloading stacks in the reftable backend refs/reftable: encapsulate reftable stack