Merge branch 'ps/reftable-get-random-fix'

The code to compute "unique" name used git_rand() which can fail or
get stuck; the callsite does not require cryptographic security.
Introduce the "insecure" mode and use it appropriately.

* ps/reftable-get-random-fix:
  reftable/stack: accept insecure random bytes
  wrapper: allow generating insecure random bytes