Content providers announce the availability of indexer data using gossip pubsub. The content providers are not connected directly to indexers, so the pubsub messages are relayed to indexers via chain nodes. This PR makes chain nodes relay gossip pubsub messages, on the /indexer/ingest/<netname> topic.
This configurability is unlocked through the `testground`
build tag, which Project Oni will uses.
Changes in the usage places of these relaxed constants
were required due to the fact that Golang constants are
untyped, but vars aren't.
Read https://blog.golang.org/constants for more info.
