Synchronized calls to Exporter::Export and Exporter::Shutdown.

[lalitb]

As of now,

1. SimpleSpanProcessor ensures that it is not possible to concurrently call Exporter::Shutdown().
2. SimpleSpanProcessor ensures that it is not possible to concurrently call Exporter::Export()
3. But it is still possible to call Exporter::Shutdown() and Exporter::Export() concurrently, and if that happens, is_shutdown/isShutdown variable defined in different exporters need to be protected.

opentelemetry::common::SpinLockMutex is lock-free, and one option could be to protect is_shutdown with that.