fix: notification issues

[chrissuozzo]

Issue
Closes #308

[lulf]

Thank you for the PR! I love the idea of introducing a GattConnection type for simplifying the API. I've left some comments inline on what changes I'd like to see.

I think in general that we should the existing GattData, GattEvent, but move the attribute server specific 'process()' into the GattConnection and/or GattConnectionEvent, to keep supporting the 'direct reply' use case without the attribute server.

[chrissuozzo]

Thanks for the feedback @lulf ! I think I've addressed everything. Two additional things have been tweaked:

1. I realized that the connect() method is actually fallible if the connections_max parameter is not properly set, so this now returns a Result<(), Error> with a newly defined Error::ConnectionLimitReached. As such, GattConnection::new() -> Self is now GattConnection::try_new() -> Result<Self, Error> and with_attribute_server() returns the same.
2. GattConnection::next() will now call-through to Connection::next() and process the GattData directly: so GattConnectionEvent::Gatt now wraps a Result<GattEvent, Error> instead of GattData. This makes the user code a little nicer (imo) and makes some sense, as when we are using a GattConnection we're putting the attribute server in charge of handling the protocol.

[lulf]

I think this looks really good, thanks again @chrissuozzo!

[lulf]

/test