delete session connections once the transport connection is made, and queued messages sent

When a new browser is opened, a temporary "session" connection is made to hold all messages until the push connection is made (in the case of polled transport this session connection is permanent).

Each session connection has an expires_at attribute, that will cause the temporary connection to be destroyed after that time.

However we should also immediately destroy the connection once the transport is established, and all queued messages are sent. Otherwise any messages sent before the temporary connection is expired will be uselessly queued up. Useless because once the transport is established the messages are simply broadcast.