AbstractConnection throws NPE when used after deserialization

[viruzzo]

In #121 Connection<A> has been made to extend Serializable, but (as noted in the comments) using any implementation of AbstractConnection<A> after deserialization fails with a NullPointerException
because it tries to use transient fields without restoring them.

Making the fields non-transient is difficult because of the larger number of classes that would need to be made serializable in turn; just restoring the fields on the other hand is not possible because there's a dependency on the API type and its parameters.

Currently the easiest path around the problem is to convert to a ConnectionData with createData(), but a correctly configured ConnectionFactoryLocator is then required to convert back to a Connection<A>.