BUG: declare and use correctly self.unique_check
I am not sure I understood the last comment in [the previous pull request](https://github.com/pydata/pandas/pull/9526), but anyway this is a rebased version.

In the previous PR I proposed to simply make the `__get__` of `is_unique` directly rely on `self.initialized`, but I am no more so sure it is a good idea, since the current status is slightly redundant but clearer (and coherent with the use of "is_monotonic").

ok, what exactly does this fix? 
 First, `self.unique_check` (which follows the same logic of `self.monotonic_check`) is now initialized.

Then, when `is_unique` is accessed, `self.unique_check` [is checked](https://github.com/pydata/pandas/blob/master/pandas/index.pyx#L272). But,`self.unique_check` [is (was) not set when the index is not unique](https://github.com/pydata/pandas/blob/master/pandas/index.pyx#L272). So on a non-unique index, every access to `is_unique` results in a call to `_do_unique_check`, which calls then `_ensure_mapping_populated`, which finally does nothing (in calls following the first) because `self.initialized` is now 1. So the performance loss is minimal, but still it doesn't make any sense.
 is  `unique_check` really a duplicate of `initialized`? if so, just remove it. check that everything passes and perf is unchanged.
 See https://github.com/pydata/pandas/pull/10229
 @toobaz just for future reference, you don't have to open a new PR every time you update. Just update your branch and push again will do!
 @jorisvandenbossche  : since I was indeed trying to understand this... by "update" do you mean "push a new commit" or "rebase and push -f"? Because I tried the latter and it didn't seem to work (but I may have been mistaken).
 As long as you stay on the same branch, it does not matter. Adding a new commit, rebasing, etc all work and the PR will be updated with that if you push (force) again.
 But I see that the new PR uses another branch name. So you only need to stay on the same branch to update the PR
 I changed branch name precisely because I didn't think using the same would work! Thanks for the tip.
