Add 2 outbound block-relay-only connections

Transaction relay is primarily optimized for balancing redundancy/robustness
with bandwidth minimization -- as a result transaction relay leaks information
that adversaries can use to infer the network topology.

Network topology is better kept private for (at least) two reasons:

(a) Knowledge of the network graph can make it easier to find the source IP of
a given transaction.

(b) Knowledge of the network graph could be used to split a target node or
nodes from the honest network (eg by knowing which peers to attack in order to
achieve a network split).

We can eliminate the risks of (b) by separating block relay from transaction
relay; inferring network connectivity from the relay of blocks/block headers is
much more expensive for an adversary.

After this commit, bitcoind will make 2 additional outbound connections that
are only used for block relay. (In the future, we might consider rotating our
transaction-relay peers to help limit the effects of (a).)

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