bgp: consider using local variables to encode FSM timers

[taspelund]

While spending a lot of time on the FSM lately, I had some thoughts on how we store and manage FSM timers:

• HoldTimer/KeepaliveTimer are specific to a connection, so they could stay in the ConnectionClock.
• IdleHoldTimer is specific to Idle, so that could be state local to FsmState::Idle.
• DelayOpenTimer is specific to Active/Connect and doesn't need to persist outside that state, so it could also be local state.
• ConnectRetryTimer really only starts/stops in Active/Connect so it could also be state local to that FsmState (technically it's started in Idle immediately before moving into active/connect, but we could just start it upon entry of the next state)