ITS#4420 hanging connections in ldap-backend

[hyc@symas.com]

I saw some hangs while tracking down ITS#4541. The problem was two (or 
more) threads trying to open the default connection at the same time on 
an LDAP* handle. I saw a few cases with 8 active worker threads, with 3 
pairs using the same session handles at once. They were all hanging, 
since no thread could read a complete message. If back-ldap was using 
TLS, things would usually crash or hang in libcrypto. With the ITS#4541 
fix I am no longer seeing these problems.

-- 
  -- Howard Chu
  Chief Architect, Symas Corp.  http://www.symas.com
  Director, Highland Sun        http://highlandsun.com/hyc
  OpenLDAP Core Team            http://www.openldap.org/project/