Improve scrutability of engine state transitions

[sam-tombury]

• Exposes transition messages via the runtime, improving observability of internal engine state (e.g. FIX connection logon status).
• Initiates transport termination where appropriate (after clean Logout, under timeout conditions, after failed CompID Logon validations as defined in the FIX Session Layer specification).
• Handles unclean shutdown, where initiated Logout message is ignored by the peer. Uses HeartbeatInt * 2 as the threshold as recommended in the FIX Session Layer specification.