client: support a 1:1 mapping with acbws and addrConns

[dfawley]

In the process of making SubConn have a String method (for debug printing), I noticed that, since the addrConn below a SubConn can change out from underneath theSubConn, there would be a race if I tried to print the addrConn's channelz ID. But, in reality, the "subchannel" that exists in channelz corresponds to our SubConns, not our addrConns, so I moved the channelz ID over to the SubChannel.

RELEASE NOTES: none

[dfawley]

It seems the thing we tried really hard to avoid doing was considerably easier than dealing with the hacks put in place to avoid doing it.

I.e. it's easier to make addrConn able to be updated than move the subchannel stats (channelz)/etc into acbw and ignore spurious updates from old addrConns once attached to acbws.

The main idea of the strategy for this is to use ac.ctx more often: previously it was canceled only when the whole addrConn was shut down. We now cancel it and recreate it whenever we want a connection attempt to stop. And since it is also canceled when the addrConn is shut down, we can replace most of the checks for ac.state == shutdown with ac.ctx.Err() != nil, because many of those uses also needed to exit when the current connection attempt was aborted.