Speed up PeerSpecificParserConstraintImpl

Speed up PeerSpecificParserConstraintImpl

This improves design by taking advantage of ClassToInstanceMap,
which provides a type-safe getInstance() method. Furthermore we
recognize that the this class is used mostly for lookups of a few
classes, we use an ImmutableClassToInstanceMap updated concurrently
via a compare-and-set.

This yields significantly faster lookups for the common cases, where
the map is empty or has a single entry. In other cases the speed up
is still there, although not as pronounced.

Change-Id: I4da36e592e88cf22d5ad1654f126eec81660c98c
Signed-off-by: Robert Varga <robert.varga@pantheon.tech>