Currently, pool::getHost() uses a fixed loop of 0-99 regardless
of how many hosts it has in it's lists so it can use a percentage
to factor in whether it should use an entry or not.
Using network regtest and a single host of 127.0.0.1:14038, hsd
burns a lot of CPU when refilling peers and this blocks incoming
requests so much it times out.

This is less noticeable the more hosts you have.

Instead, use shorter loop of 10.

This brings CPU utlisation on my NetBSD Xen DOMU on regtest with
one host from 100% to 6% and connecting 4 to peers on testnet
from 85% to 8%, allowing RPC commands to now work.

Fixes handshake-org#220.

This commit removes the discover external check in the pool. Currently
we already store all local addresses that are sent to us from peers in
the hostlist, so there is no reason to rely on an external service to
check for our local/external address. This was removed from bitcoin in
commit 845c86d.

This commit adds the getGroup function to net addresses. It allows for
network addresses to be grouped into buckets, such that we can limit the
number of outgoing connections per group to a certain amount. This will
allow for increased connection diversity and reduced attack surface
area.

Co-authored-by: Nodari Chkuaselidze <nodar.chkuaselidze@gmail.com>

This commit checks the outbound peers to see which group their network
addresses lives in before connecting. By doing this, we prevent a node
from connecting to all outbound addresses in a very close network group
bunch. This helps prevent someone spinning up multiple nodes on a
similar network e.g. AWS, GCP and using those to fill other nodes'
outbounds.

This commit just adds 2 operations in both addOutbound and addLoader. It
first checks if the new address pull from hostlist belongs in the same
group as an already connected peer. If it does not, it precedes as
usual, and if the connection succeeds then it adds that new outbound
group to the list of groups currently connected to.

Co-authored-by: Nodari Chkuaselidze <nodar.chkuaselidze@gmail.com>