FCPConnectionHandler: Resolve FIXME of dealing with nulled WeakReferences
- This is implemented as opportunistic garbage collection instead of using a reference queue for the reasons explained in https://github.com/xor-freenet/fred-staging/commit/882e1a150b785d11a3ca8f35a26aec6d692d6176
- This also adds a comment about the opportunistic part which points the user to the location of the JavaDoc which is linked above