[OTR] A number of improvements for libotr 4.x support.

* Fix problem of initialization of private conversation when both sides use
  libotr 4.0.x. Protocol OTRv2 is forced. This is temporary solution.
* Fix tiny memory leak when libotr 4.x is used.
* Tiny improvement in endSession() function when libotr 4.x is used.
* Better error handling when libotr 4.x is used.