Merge pull request #210 from mulesoft/MULE-7149

MULE-7149 - fix flow variable propagation between flows using flow-ref(cherry picked from commit 4ec607bb3f8fba7b821415f9bf2e49aeffb215f7)
(cherry picked from commit fe423b35bc58b9620d6c6b83518a57f3c40bebd3)

Conflicts:
	core/src/main/java/org/mule/DefaultMuleEvent.java
Merge pull request #217 from mulesoft/MULE-7149

MULE-7149 - Fixing test failures - Making flow-ref to use the same sessi...(cherry picked from commit 1965727375edf3ce19b153c52ba022a0e8afbfc2)
(cherry picked from commit ecd782864babcacac67b7cc88c5bc153c9982e00)