Refactor choose_good_entry_server so it is (almost) never used

From my prop271 branch:

 * XXXX prop271 this function is used in four ways: picking out guards for
 *   the old (pre-prop271) guard algorithm; picking out guards for circuits;
 *   picking out guards for testing circuits on non-bridgees;
 *   picking out entries when entry guards are disabled.  These options
 *   should be disentangled.

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