reftable/system: introduce `reftable_rand()`

reftable/system: introduce `reftable_rand()`

Introduce a new system-level `reftable_rand()` function that generates a
single unsigned integer for us. The implementation of this function is
to be provided by the calling codebase, which allows us to more easily
hook into pre-seeded random number generators.

Adapt the two callsites where we generated random data.

Signed-off-by: Patrick Steinhardt <ps@pks.im>
Signed-off-by: Junio C Hamano <gitster@pobox.com>