Session event loop

[bhaan]

The current session event loop has a primitive approach toward handling outgoing messages sent from application code. A call to quickfix.SendToTarget essentially adds the message to a queue, which the event loop picks up during it's next pass. This is limiting for applications that want to account for a message send failure, as there is no way for the event loop to communicate that back to application code.

These commits propose the following changes:

• Remove the toSend message queue
• Add a dedicated channel in the event loop for sending messages.
• Have SendToTarget push messages to the event loop, and block on a corresponding error channel
• Force the FromAdmin/App callback to occur on a dedicated goroutine. This creates separation between "application space" and "session space", and is required to prevent deadlocks on calls to SendToTarget during the FromAdmin/App callback.
• Separate session state message handling into three parts: 1) Vaildation 2) FromAdmin/App callback 3) Processing for admin and rejected messages. This is required to enable the above changes.

[daino3]

any ideas on what the compiler will make of these stucts / anonymous functions getting continuously redefined from within run?

[bhaan]

the size of golang structs I believe are just 8 bytes in addition to the types they enclose. So every time you'd call run, you'd be allocating, on the stack, 8 bytes + size of Message + size of MessageRejectError. Which is negligible considering when and how many times run is expected to be called.

[cbusbey]

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