Root cause of #2493.

When accessing a constant like a::B, we attempt to cache the value against module a's id and the global constant invalidator. Unfortunately, for cases where this form may see many different "a" values (as is the case in #2493) we fail to cache the value but still keep trying. This leads to tremendous overhead in method handle creation and call site binding/rebinding.

For the moment I'm disabling this caching. JRuby 1.7 did not do it, so I do not have the heuristics in hand to tune these access sites. We will endeavor to fix this before final.