Multithreading with MultiDBClient and the XTypes::Registry

[pierrewillenbrockdfki]

When using MultiDBClient in a multithreaded program, the Information stored behind XTypePtrs can be observed to change during MultiDBClient::find.

This is due to the Servers needing to provide a list of XTypePtrs, thus have to try to fill their version of the data for an URI during ::find.

This only happens when the same URI is reachable by multiple Servers.

Sequence of events:

1. MultiDBClient::find begins
2. MultiDBClient::find queries first Server having a version of the conflicting URI
3. All threads see the data behind the XTypePtr for the conflicting URI change to the version of the first(lower priority) server. This data is incorrect.
4. MultiDBClient::find queries second Server having a version of the conflicting URI
5. All threads see the data behind the XTypePtr for the conflicting URI change to the version of the second(higher priority) server
6. ...
7. MultiDBClient::find ends

For now, this is a theoretical issue. Access currently is not multithreaded.