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An asynchronous SQL statement.
This differs from mozIStorageStatement by only being usable for asynchronous execution. (mozIStorageStatement can be used for both synchronous and asynchronous purposes.) This specialization for asynchronous operation allows us to avoid needing to acquire synchronization primitives also used by the asynchronous execution thread. In contrast, mozIStorageStatement may need to acquire the primitives and consequently can cause the main thread to lock for extended intervals while the asynchronous thread performs some long-running operation.