Fix issue preventing the parsing of certain async messages

When multiple asynchronous handshake messages are concatenated into the same record message we would fail parsing them. This change ensures that we can parse them correctly, and at the same time that we can send multiple new session tickets on a single go.

Note however that the latter is not a functional test of the fix as our lower level buffering does not combine multiple handshake messages into a single record message.

Checklist

• Code modified for feature
• Test suite updated with functionality tests

Reviewer's checklist:

• Any issues marked for closing are addressed
• There is a test suite reasonably covering new functionality or modifications
• Function naming, parameters, return values, types, etc., are consistent and according to CONTRIBUTION.md
• This feature/change has adequate documentation added
• No obvious mistakes in the code