Expose TypedReadTransaction.exists(InstanceIdentifier)

Expose TypedReadTransaction.exists(InstanceIdentifier)

This operation was not exposed at the binding layer when these
utilities were conceived and thus is lacking here, even when they
are available as part of MDSAL-389.

The MD-SAL version of TypedReadTransaction has been updated back
then, this patch backports these changes to legacy Genius APIs.

Also converts a few callers to use this capability, potentially
improving performance.

JIRA: MDSAL-389
Change-Id: Ieb870a49c8239ee74e697d7d36dea431b24362ae
Signed-off-by: Robert Varga <robert.varga@pantheon.tech>
(cherry picked from commit 7144e4db6a02daf37f8ba055745a359d0a80c21e)