Fix potential deadlock when deleting queue

[robberlang]

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This fixes Issue #758 by using an upgrade lock.

As well, it makes RPCService::send_message thread-safe by using atomic int, and removes a redundant call to TaskExecutionThreadPool::create_queue and uses std::unique_ptr rather than the unnecessary shared_ptr for the request queue in RPCService.

[AmplabJenkins]

Can one of the admins verify this patch?