Fix 1691 : OData v4 Serialization Perf is 10% slower than v3

Based on profiling, found out that if an OData route prefix is null, the
existing ODataRoute.GetVirtualPath method does not call
GenerateLinkDirectly to generate an optimized direct link, instead
base.GetVirtualPath is invoked.
Migrate To ODL 6.4
Fix 1937 : Expanded navigation properties should be properly represented
on the response payload

Serialize null single valued navigation property as null in payload.
Serialize null collection valued navigation property as empty collection
in payload.  Previously, they were not serialized, i.e., missing in
payload.
Remove application start code.
Removed app start code from FB tests.
[RTM]1681:enforce $ref and $value be the last segment
Add test cases to verify the behavior after migration to ODL 6.4
[RTM]1974: The function parameter translated by ODataPathSegmentTranslator
is always null when its EdmType is nullable
Add test cases to verify the fix in ODL after migration to ODL 6.4