Separated SUT into a reader and a writer,

in preparation for migrating the test away from AtomEventStream<T> over to
FifoEvents<T>.
Migrated a test from AtomEventStream<T>

to a combo of AtomEventObserver<T> and FifoEvents<T>, since
AtomEventStream<T> will soon be deprecated.
Separated SUT into a reader and a writer,

in preparation for migrating the test away from AtomEventStream<T> over to
FifoEvents<T>.
Migrated a test from AtomEventStream<T>

to a combo of AtomEventObserver<T> and FifoEvents<T>, since
AtomEventStream<T> will soon be deprecated.
Deprecated AtomEventStream<T>.