Avoid counting failed connect attempts when probably offline.

If a node is offline failed outbound connection attempts will crank up
the addrman counter and effectively blow away our state.

This change reduces the problem by only counting attempts made while
the node believes it has outbound connections to at least two
netgroups.

Connect and addnode connections are also not counted, as there is no
reason to unequally penalize them for their more frequent
connections -- though there should be no real effect from this
unless their addnode configureation is later removed.

Wasteful repeated connection attempts while only a few connections are
up are avoided via nLastTry.

This is still somewhat incomplete protection because our outbound
peers could be down but not timed out or might all be on 'local'
networks (although the requirement for multiple netgroups helps).