Separate out single-key Identifiable handling

Separate out single-key Identifiable handling

We can further optimize IdentifiableItemCodec by separating
out the common case of a single-key from multiple-keys.

This allows us to specialize ImmutableMapTemplates we use for
the two cases, more efficiently binding to
NodeIdentifierWithPredicates constructor.

Furthermore we can specialize key constructor invocation such
that we directly invoke the constructor for single-key identifiers,
without the need to iterate and allocate arrays.

JIRA: MDSAL-387
Change-Id: I2fc26dc7285bfb3fe829b8b0efd9937daf6fbc8d
Signed-off-by: Robert Varga <robert.varga@pantheon.tech>