BUG-1384: YangStoreServiceImpl.refresh() should never block

As descirbed in the bug, we are seeing large amounts of contention on
the monitor when calling refresh(). Rework the safety such that the
refresh path does not block and perform checking for refresh in the
get() path, which is forced to retry should a refresh occur.

With this patch reported contention disappears completely and startup
time is improved by about 8 seconds on stock SP edition.

Change-Id: I4c6109d77324d3d1b700041b54a95dc346e3d372
Signed-off-by: Robert Varga <rovarga@cisco.com>