Release GIL when polling Runtime API for next invocation

[carlzogh]

Notes

runtime_client is holding the GIL (Global Interpreter Lock) while it is polling for the runtime/invocation/next invocation, which is meant to be an async I/O operation.

According to the Python extension docs on Initialization, Finalization, and Threads, any blocking I/O should release the lock explicitly to allow other code to continue. This is especially true for extensions where developers (us, in this case) should manage releasing/reacquiring the lock.

Without the lock, even the simplest operations could cause problems in a multi-threaded program: for example, when two threads simultaneously increment the reference count of the same object, the reference count could end up being incremented only once instead of twice.

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[zsombor-balogh]

LGTM!