Results of ReadableState.read() should be snapshots of the underlying state

[kennknowles]

Future modification of state should not be reflected in previous calls to read(). For example:

@stateid("tag") BagState<Integer> state;
Iterable<Integer> ints = state.read();
state.add(17);
// ints should still be empty here.

Imported from Jira BEAM-2975. Original Jira may contain additional context.
Reported by: millsd@google.com.