Leaked HTTP input connection between agent and IOSwitchboard when launched with TTY enabled.

Steps to reproduce:
1) Launch a TTY container.
2) Send the `ATTACH_CONTAINER_INPUT` request to the agent via an HTTP connection.
3) Close a tcp socket used to send `ATTACH_CONTAINER_INPUT`.
4) Send another `ATTACH_CONTAINER_INPUT` request to the agent - it returns `409 Conflict` HTTP error.

For each incoming `ATTACH_CONTAINER_INPUT` request the agent creates an HTTP connection to the IOSwitchboard via unix socket. This connection is used to retransmit client requests to the IOSwitchboard. IOSwitchboard closes this connection automatically once the client closes its HTTP connection to the agent: for more details see HTTP handlers in the agent and in the IOSwitchboard.
IOSwitchboard does not allow multiple input connections.
Currently, IOSwitchboard does not close HTTP connection for the `ATTACH_CONTAINER_INPUT` in the case described above. Hence, IOSwitchboard returns an error for the subsequent attempts to attach to the container input. The root cause needs to be found.