context checking and multiple match resolution via an internal search
b) move the candidate listing in a persistent per-thread buffer
c) fix bind procedure
d) minor cleanup

(a) and (b) should address ITS#2935; (b) and (c) should address ITS#3171

A test is also added

  - recovers correctly, logs status
  - for Quick mode, where no transactional environment exists, refuses to
     start if the database is inconsistent
  - warns of poor performance if DB_CONFIG is not present
  - detects changes to DB_CONFIG and forces a recovery

fix previous commit in case of access to non existent backend (e.g. rootDSE); add (and document) authzID/DN support