## Proposed Changes
Adds a gossipsub topic filter that only allows subscribing and incoming subscriptions from valid ETH2 topics.
## Additional Info
Currently the preparation of the valid topic hashes uses only the current fork id but in the future it must also use all possible future fork ids for planned forks. This has to get added when hard coded forks get implemented.
DO NOT MERGE: We first need to merge the libp2p changes (see https://github.com/sigp/rust-libp2p/pull/70) so that we can refer from here to a commit hash inside the lighthouse branch.
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