Improve performance of slide histogram

Partition slide histogram reservoir by erlang:system_info(scheduler_id),
as previously done with counter and spiral metrics.
Fix EUnit failures that should've been fixed before merging jdb-faster-slide

* Avoid EUnit-enforced timeouts with prop_window_test_()
* Change folsom_sample_slide_test.erl to avoid failure like https://gist.github.com/272dc6bcd2478484bffa
* Fix sorting bug Russell sees but I don't, sorry!
Merge branch 'jdb-faster-slide-eunit-fixes-squashed'