PrepareMessages in WebSocket code can be made a GraphStage or splitAfter

**Issue by [ktoso](https://github.com/ktoso)**
_Thursday Mar 24, 2016 at 19:08 GMT_
_Originally opened as https://github.com/akka/akka/issues/20136_

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We had it implemented using splitWhen, but we could actually implement it as graph stage or splitAfter, to be decided which way is cleaner - perhaps the graph stage actually - as it could collapse many steps into one?

See here: https://github.com/akka/akka/pull/20089/files/864f342f4c0da02420225259d154e7c293b71f6e#diff-d7697d316b990df80d5e2d4308e7b93fR119

Start with the PrepareMessages implementation in the WebSocket code referenced by the linked pull request, then compare the existing splitWhen approach with splitAfter and a GraphStage. Determine which design is cleaner and reduces the processing steps; done means the chosen refactor preserves the current WebSocket message preparation behavior.

Written by the indexing model from the issue text.