Fix issues with raft traffic encryption (#1919)

and run raft behave tests with encryption enabled.

Using the new `pysyncobj` release allowed us to get rid of a lot of hacks with accessing private properties and methods of the parent class and reduce the size of the `raft.py`.

Close https://github.com/zalando/patroni/issues/1746

requirements.txt

@@ -9,6 +9,7 @@ python-consul>=0.7.1
 click>=4.1
 prettytable>=0.7
 python-dateutil
-pysyncobj>=0.3.7
+pysyncobj>=0.3.8
+cryptography>=1.4
 psutil>=2.0.0
 ydiff>=1.2.0