The enhancement in PR #2367 removed the `"@isDoc"` check,
which was the main difference between the legacy and current XML schema.
Reducing the XPath to `"//*[@key=$guid]"` will perform the same for both types of schema.
_(and saves on a couple of allocations)_
This is a proposal for a temporary workaround to issue U4-10756.
It uses a local static ConcurrentDictionary to store a lookup of Guid/int values.
If the Guid isn't in the lookup, then the traditional XPath is used, which would add the resulting node ID (int) to the lookup.
If the lookup contains the Guid, then the returned int value will be used to perform a more efficient retrieval.
<http://issues.umbraco.org/issue/U4-10756>
