Please review (take 2): [389 Project] #351: use betxn plugins by default

[Noriko Hosoi]

https://fedorahosted.org/389/ticket/351

https://fedorahosted.org/389/attachment/ticket/351/0001-Trac-Ticket-351-use-...

Fix description:
  In addition to the fixes following the Rich's suggestions (comment 11),
  the take 2 patch contains ...
  . to register cos_post_op are POSTOP only to avoid the deadlock between
  the transaction and cos change_lock.
  . to move transaction begin before "finding" an backend entry in
  ldbm_back_add/delete/modify/modrdn to avoid the deadlock between the
  serial lock and the individual entry cache lock.