schedulers must call parent de/activate methods

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#	modified:   master/buildbot/schedulers/base.py
#	modified:   master/buildbot/schedulers/basic.py
#	modified:   master/buildbot/schedulers/dependent.py
#	modified:   master/buildbot/schedulers/timed.py
#	modified:   master/buildbot/schedulers/trysched.py
#	modified:   master/buildbot/test/util/scheduler.py
#	modified:   master/docs/developer/schedulers.rst
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don't assert on JSONRPC error message contents, which depend on JSON parser