When we migrated to the updated autopilot library in Nomad 1.4.0, the interface
for finding servers changed. Previously autopilot would get the serf members and
call `IsServer` on each of them, leaving it up to the implementor to filter out
clients (and in Nomad's case, other regions). But in the "new" autopilot
library, the equivalent interface is `KnownServers` for which we did not filter
by region. This causes spurious attempts for the cross-region stats fetching,
which results in TLS errors and a lot of log noise.
Filter the member set by region to fix the regression.
Tim Gross