Optimize BuilderUtils

- reuse Date(0L)
- skip empty strings in SLASH_SPLITTER
- check just the first character to determine if XPath is absolute

Change-Id: Ia4a4d30ebe5e4d2e3866761a4393ebf564f6cf38
Signed-off-by: Robert Varga <rovarga@cisco.com>
Reuse a single QNameModule for NULL

QNameModules with null namespace/revision are an easy pick -- reuse a
single instance for all of them.

Change-Id: I1f519ad5a46c82da3daf17122087af89c97e5df8
Signed-off-by: Robert Varga <rovarga@cisco.com>