Concurrent use of the same Query instance is not always threadsafe

[tzaeschke]

When multiple threads execute the same query using the same Query instance, a race condition can occur during initialization of query execution parameters.

No race condition occurs when the query does not use parameters.

As suggested by JDO, the query executor should use ThreadLocal query parameters to allow parallel query execution.

TODO:

• Only do this for setMultiThreaded(true) ?
• Provide default parameter set? At least give good error message if query is executed from a thread that did not set parameters (e.g. they were set in a different thread)
• Include also "range" and other execution settings
• Consider synchronizing the query compilation, while we are at it?