Deterministic timeout

Picking up a theme from #472 and #520 which warrants its own issue: It seems useful to reimplement the RealmOptions.timeout heuristic by counting evaluation operations, over the current nondeterministic approach of measuring wall-clock time.

@NTillmann per your earlier comments I gather you think Environment.prototype.evaluate would the right place to increment and check this proposed counter? If so, seems straightforward enough and I'll give it a go.