Greengrass IPC

[yourslab]

• Fixes the definition of the flush callback along with futures returning the expected result of the operation
• Introduces a few mechanisms such as ProtectedPromise and mutexes to handle synchronization across multiple threads
• Avoids usage of pointers wherever possible in place of references

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.

[yourslab]

Note that this type will never be public facing and is just a helper class. Ultimately, we still return an std::future.

[yourslab]

Closing for #273