In this commit, we fix the peer selection for the pruned block
dispatcher. Turns out that the bits are actually mutually exclusive: a
node can set the network bit _and_ the limited (pruned bit).
The prior logic would only seek out "pure" nodes, meaning older nodes
that wouldn't set the limited bit when they're a full-full node. We fix
this by ignoring the limited bit, and we'll only look at the network and
segwit bits.
A unit test has also been added to show the issue in the prior filter
function.
It's possible for bitcoind instances to only have connections to pruned
nodes after its initial block download, which are incompatible with the
PrunedBlockDispatcher. This would result in GetBlock requests for pruned
blocks to never resolve. Since bitcoind also exposes a GetNodeAddresses
RPC, which returns random reachable addresses from its address manager,
we can leverage it to obtain a new candidate set of peers that we
otherwise wouldn't obtain through GetPeers.
To minimally support wallets connected to pruned nodes, we add a new
subsystem that can be integrated with chain clients to request blocks
that the server has already pruned. This is done by connecting to the
server's full node peers and querying them directly. Ideally, this is a
capability supported by the server, though this is not yet possible with
bitcoind.
