Bug 2378: Duplicates in selected peer destinations (#112)

Bug 2378: Duplicates in selected peer destinations (#112)

Duplicates in FwdServers lead to excessive peer connection retries, skew
in round-robin peer selection, and probably other problems.

This bug was fixed in 2008 but that v2 fix was never ported to v3. This
fix includes a bug 2408 fix for the original (bug 2378) fix, although I
adjusted bug 2408 logic to explicitly reject duplicate PINNED
destinations and to clarify why PINNED connection handling is "special".

I also centralized and improved peerAddFwdServer-related debugging,
removing duplicated and slightly inconsistent code.