Improve connection state handling

[adamfowleruk]

Android and C++ API implement a state machine to manage outgoing connections on those platforms. On iOS we've been relying on the iOS Bluetooth event mechanism. It turns out that mechanism has become much less reliable than needed, and raises many more events, causing a multitude of connection attempts which in turns leads to unreliability of older, less performant Android devices.

Consider a temporary patch to work around a connection already being requested, followed by porting the state machine from Android to iOS for greater reliability.

[adamfowleruk]

Fixed by #191.