Cherry pick commit from PR #367 , Update changelog , bump version to 2.2.0rc4

Author: dhoomakethu

CHANGELOG.rst

@@ -6,6 +6,7 @@ from pymodbus.client.asynchronous import ModbusTcpClient
 ```
 
 * Support Python 3.7
+* Fix to task cancellations and CRC errors for async serial clients.
 * Fix `AttributeError` when setting `interCharTimeout` for serial clients.
 * Provide an option to disable inter char timeouts with Modbus RTU.
 * Add support to register custom requests in clients and server instances.

pymodbus/version.py

@@ -41,7 +41,7 @@ def __str__(self):
         return '[%s, version %s]' % (self.package, self.short())
 
 
-version = Version('pymodbus', 2, 2, 0, "rc3")
+version = Version('pymodbus', 2, 2, 0, "rc4")
 
 
 version.__name__ = 'pymodbus'  # fix epydoc error