Fix: Get cccd with FIND_INFORMATION instead of READ_BY_TYPE

[simonft]

Fixes #346

[simonft]

A couple notes:

I changed get_characteristic_cccd() to no longer return the state of the CCCD from the server because it would require sending an additional message and the value wasn't actually being used anywhere. Happy to change that back though.

I implemented an arm for encode() (copying the format of the ReadByType arm) because it was throwing an error otherwise. I don't understand what that method is for though. Both arms generate bytes from their associated iterators, which parse from bytes. So they end up converting from bytes into messages and then immediately back into those same bytes. If I'm understanding it correctly, attribute_server.rs is where the serialization occurs when response are being sent to clients.

[lulf]

/test