When the fascist_firewall_choose_address_ functions don't find a
reachable address, set the returned address to the null address and port.

This is a precautionary measure, because some callers do not check the
return value.

Fixes bug 24736; bugfix on 0.2.8.2-alpha.

This makes clients on the public tor network prefer to bootstrap off fallback
directory mirrors.

This is a follow-up to 24679, which removed weights from the default fallbacks.

Implements ticket 24681.

This way it will match the insert queue in 029 and later.

We've been seeing problems with destroy cells queues taking up a
huge amount of RAM.  We can mitigate this, since while a full packed
destroy cell takes 514 bytes, we only need 5 bytes to remember a
circuit ID and a reason.

Fixes bug 24666. Bugfix on 0.2.5.1-alpha, when destroy cell queues
were introduced.

Fixes bug 24633; bugfix on 0.2.9.1-alpha.

Fixes bug 24480; bugfix on 0.2.5.16.

"ours" merge to avoid version bump.

"ours" merge to avoid version bump.

"ours" merge to avoid version bump.

"ours" to avoid version bump