PROTON-2792: [C++] check that scheduled tasks are active under lock

[astitcher]

Previously we checked whether the tasks were active without locking which was bad.

This is is an alternative fix (which has the correct semantics) to #421.

I think it probably still needs a test for the fixed behaviour and the correct semantics before merging. This would be new tests in container_test.cpp

[zlomek]

Yes, this fixes the issue while preserving the semantics, thank you.

[cliffjansen]

+1 I will merge this.