This gives KnownAddress a sync.RWMutex so the exported methods may
safely access the na (*wire.NetAddress) and lastattempt fields.
The AddrManager is updated to lock the new KnownAddress mutex before
assigning to na or lastattempt.
The other KnownAddress fields are only accessed by AddrManager, using
its own Mutex for synchronization.
This change was suggested as Countermeasure 2 in Eclipse Attacks on
Bitcoin's Peer-to-Peer Network, Ethan Heilman, Alison Kendler, Aviv
Zohar, Sharon Goldberg. ePrint Archive Report 2015/263. March 2015.
This mimics Bitcoin Core commit
f68ba3f67bd500a64fb8932c6b41924ddc31d76f
