OpenAPI: Iterate over models in PathsStream

Use originally intended iterative approach to get next chunk of paths (for one model) in PathsStream. The way that we will have:

new PathsEntity(toPaths(iterator.next()))
...
private Deque<PathEntity> toPaths(final Module module)

This will restore original draft idea presented in https://git.opendaylight.org/gerrit/c/netconf/+/108258 and possibly saves even more memory as we did for NETCONF-938. After work is done check performance with 2GiB heap space and 6 cores as is described in NETCONF-1234.