FELIX-3910: Added new SerialExecutor, which blocks other invokers while a leader is executing a task.
The executor is reentrant: when one leader executing thread reschedule some runnables, the tasks are executed directly
in the leader caller thread.


git-svn-id: https://svn.apache.org/repos/asf/felix/trunk@1535805 13f79535-47bb-0310-9956-ffa450edef68
FELIX-3910: Made final or volatile some class attributes, which are set from synchronized setters,
but possibly read from unsynchronized getters.


git-svn-id: https://svn.apache.org/repos/asf/felix/trunk@1535807 13f79535-47bb-0310-9956-ffa450edef68