G1: Replace collector state booleans with explicit state variable(s)

The current state of the G1 collector is encapsulated in the g1CollectorState class, but the state is scattered over a set of 12 booleans. The actual number of legal states is less than 10, so there are 4000+ illegal combinations of those booleans :-)

The state should be replaced by one (or a few) proper state variables with a proper state machine managing legal transitions between states.