Improve naming in order entry server

Rename 'sessions' to 'toKeepAlive' to emphasize that the order entry
server tracks sessions only for the purpose of keeping them alive.
Improve connection acceptance

Ensure that a connection is successfully registered to the selector
before adding the session to the keep-alive list.
Close session on transmission error
Terminate session on logout request
Share outbound message templates between sessions

Reduce memory usage in the trading system by sharing outbound message
templates between sessions.
Merge pull request #9 from jvirtanen/fixes/session-handling

Improve session handling in trading system