Add ability to update connection params after connect

[dsanders11]

@sandeepmistry, take a look when you get a chance.

I ran into an issue when using Web Bluetooth with Google Chrome, where after a few messages Chrome would send a connection params update to change the connection interval to a very slow 97.5 ms, which is too slow for my application. After that happens if I change them back to what I have them set at for new connections, Chrome accepts it and doesn't mess with it any further.

This PR adds two things, ability to update the connection params during an active connection, and a new event so you can detect when the connection params are changed by the central.