fix_constant_emit_in_statefulapply

[jingjingwang]

If a StatefulApply has a constant emitter (e.g. sin(-1)), it still constructs a GenericEvaluator for it, which is not compiled (due to being a constant). Then the evaluator is null when eval() is called. Instead, a ConstantEvaluator should be constructed (same as in Apply), which compiles in its constructor. A counter example is:

APPLY counter() { [0 AS c]; [c + 1]; c; }; R = scan(TwitterK); R_seq = select R.*, sin(-1) as t, counter() as c from R;

[coveralls]

Coverage decreased (-0.04%) to 27.364% when pulling 838aac0 on fix_constant_emit_in_statefulapply into b61a318 on master.

[coveralls]

Coverage decreased (-0.04%) to 27.365% when pulling 7ccc8e2 on fix_constant_emit_in_statefulapply into b61a318 on master.