Merge branch 'ps/reftable-fixes-and-optims'

More fixes and optimizations to the reftable backend. * ps/reftable-fixes-and-optims: reftable/merged: transfer ownership of records when iterating reftable/merged: really reuse buffers to compute record keys reftable/record: store "val2" hashes as static arrays reftable/record: store "val1" hashes as static arrays reftable/record: constify some parts of the interface reftable/writer: fix index corruption when writing multiple indices reftable/stack: do not auto-compact twice in `reftable_stack_add()` reftable/stack: do not overwrite errors when compacting