Fix YangStoreServiceImpl safety model

Tracing SP edition startup shows that the fileinstall thread is blocked for 22 seconds (out of total 65 second bootup) trying to call YangStoreServiceImpl.refresh().

Since all refresh() does is invalidate the cache, it should be possible to add just a volatile flag, which would signal the constructing thread (which needs to be synchronized), that it should throw away the result and retry.