RepositorySimulator: implement non-consistent snapshot

[sechkova]

Description of issue or feature request:
Repository simulator currently does not implement non-consistent snapshot mode.

Current behavior:
NotImplementedError("non-consistent snapshot not supported")

Expected behavior:
Able to serve data with consistent snapshot turned off.

[sechkova]

A related but different issue that can be better tested after implementing non-consistent snapshot is serving snapshot/targets data with wrong version. See #1615 (comment).

The two issues can be solved together, testing if the proposed fix of #1615 causes any problems with other tests.