Tweak network policy slightly for MsgAnnounceBlockHeader
Tweak network policy slightly for MsgAnnounceBlockHeader

We were observing errors in the logs of the form

  Rejected alternative as it has 0 messages ahead and 1 messages in
  flight, which total more than the maximum 0` for
  `MsgAnnounceBlockHeader`, from core to relay.

This is due to the fact that block requests and header announcements
had the same precedence. So this change bumps the precedence of header
announcements up to highest.

We originally had them at different levels because we want to start
sending headers to other core nodes before sending them to relay nodes
but this is not a big deal since they can all happen concurrently and
there is a low limit on concurrency.

input-output-hk/cardano-sl,input-output-hk/pos-haskell-prototype,input-output-hk/cardano-sl,input-output-hk/pos-haskell-prototype,input-output-hk/pos-haskell-prototype,input-output-hk/cardano-sl,input-output-hk/cardano-sl