Fix NPE for SocketConnection

[kwvanderlinde]

Identify the Bug or Feature request

Fixes #5558

Description of the Change

More clearly distinguishes the "connected" state from other states for SocketConnection. socket, send, and receive are only accessed if socket is first checked to not be null. hostName and port are only accessed if socket is first checked to be null.

Also refactored AbstractConnection a bit:

• Require the ID to be provided in the constructor (guarantees it is a constant value, and one less method for child classes to implement)
• Moved the readMessage() and writeMessage() out of AbstractConnection into SocketConnection and WebRTCConnection since each one is specific to one of those.

Possible Drawbacks

Shouldn't be any.

Documentation Notes

N/A

Release Notes

• Fixed a null pointer exception in socket connections.

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