Refactor relay-picking logic to exist in a more central form

Our relay selection logic exists in tor-circmgr: except of course when it's in guardmgr...

...and the code to pick a relay at random is just in NetDir...

...unless we're talking about a fallback directory, which is in guardmgr.

This situation is a bit complicated, and it will get worse when we implement bridges in %Arti 1.1.0: Anticensorship ready . Let's try to get it resolved.

This comes out of a discussion at !583 (comment 2812681)