server/fsm: support parallel connections

Support two parallel connections; outgoing or incoming; until
established state.

When trying to establish a BGP session, the current implementation
keeps only one TCP conneciton; outgoing or incoming. This makes the
session establishment fragle.

RFC 4271 says:

6.8. BGP Connection Collision Detection

If a pair of BGP speakers try to establish a BGP connection with each
other simultaneously, then two parallel connections well be formed.

Although RFC 4271 can be interpreted as allowing multiple incoming
connections, to keep the implementation simple, we only manage two
simultaneous connections; one incoming and one outgoing.

Each connection (incoming and outgoing) maintains its own FSM state,
resulting in two concurrent states during connection establishment.
When an API queries the FSM state, the implementation returns the
passive (incoming) connection's state until one of the connections
reaches OPENCONFIRM.

This behavior is implementation-dependent. For example, BIRD returns
whichever connection has progressed further in the FSM state machine.

Signed-off-by: FUJITA Tomonori <[email protected]>

Author: fujita