runtime/src/enclave_rpc/client: Fix panic on drop in async context

[peternose]

The graceful shutdown of active sessions was removed, as they should not be closed when the RPC client is dropped. Instead, we should explicitly invoke the appropriate functions.

Reasons:

• Blocking is not acceptable because drops should be fast, and sessions should be closed asynchronously.
• Spawning a new task is also not acceptable since drops should (mostly) be deterministic. There could be issues if the code following an implicit drop call relies on sessions being closed (not a problem yet).
• If the Tokio runtime stops and the async tasks spawned during the drop haven't completed, they will be destroyed without being executed.

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