Merge branch 'ps/reftable-concurrent-writes'

Give timeout to the locking code to write to reftable.

* ps/reftable-concurrent-writes:
  refs/reftable: reload locked stack when preparing transaction
  reftable/stack: allow locking of outdated stacks
  refs/reftable: introduce "reftable.lockTimeout"

diff --cc refs/reftable-backend.c
Simple merge

diff --cc t/unit-tests/t-reftable-stack.c
Simple merge