Collection broadcast loses message data due to moves when promoted to CollectionMessage

[lifflander]

Describe the bug
When a broadcast to a collection occurs using a normal vt::Message (not a collection message) and it is serialized, the message is de-allocated preemptively before all the collection elements on a rank receive it (after the first one does). Due to a template bug in proxy.broadcast it won't actually accept a vt::Message (it looks for a type in vt::Message that doesn't exist). So the only way to get it to fail is by using a callback that targets a collection as a broadcast.

To Reproduce
Steps to reproduce the behavior:

1. Run examples/callback/callback.cc

Expected behavior
A clear and concise description of what you expected to happen.

Screenshots
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Platform (please complete the following information):

• Supercomputer or desktop [e.g., Trinity, Sierra)
• Compiler environment [e.g., clang-4.0, gcc8]
• Libraries [e.g., mpich-2, openmpi)
• Commit ID or release of VT [e.g., vt-1.0.0]

Additional context
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