Decouple ..attach_circuit() from most of its callers.

Long ago we used to call connection_ap_handshake_attach_circuit()
only in a few places, since connection_ap_attach_pending() attaches
all the pending connections, and does so regularly.  But this turned
out to have a performance problem: it would introduce a delay to
launching or connecting a stream.

We couldn't just call connection_ap_attach_pending() every time we
make a new connection, since it walks the whole connection list.  So
we started calling connection_ap_attach_pending all over, instead!
But that's kind of ugly and messes up our callgraph.

So instead, we now have connection_ap_attach_pending() use a list
only of the pending connections, so we can call it much more
frequently.  We have a separate function to scan the whole
connection array to see if we missed adding anything, and log a
warning if so.

Closes ticket #17590

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