Avoid the "maximum recursion depth exceeded" RuntimeError with nested specifications

[fogathmann]

We currently traverse specification trees during evaluation recursively, which, for very deeply nested criteria, leads to the infamous RuntimeError: maximum recursion depth exceeded exception. This needs to be fixed.

[fogathmann]

The following test case, to be added to the everest.tests.test_querying.EvalExpressionTestCase test class demonstrates the problem:

    def test_many_criteria(self):
        red_fn = lambda x, y: x | ValueEqualToFilterSpecification('id', y)
        expr = EvalFilterExpression(
                    func_reduce(red_fn,
                                range(1, sys.getrecursionlimit() + 1),
                                ValueEqualToFilterSpecification('id', 0)))
        ent0 = MyEntity(id=0)
        ent1 = MyEntity(id=sys.getrecursionlimit() + 2)
        ents = [ent0, ent1]
        self.assert_equal(list(expr(ents)), [ent1])

The test case needs the following two imports:

from functools import reduce as func_reduce
import sys