Fix bad bug in folly::ThreadLocal

Fix bad bug in folly::ThreadLocal

Summary:
There were a few bugs, actually:

1. prevSize < jemallocMinInPlaceExpandable compared an element count with
a byte size; this hid the next bug for a while (as we needed 4096
ThreadLocalPtr objects in order to trigger it)

2. if rallocm(... ALLOCM_NO_MOVE) succeeds in expanding in place, we don't
increment elementsCapacity_, which is bad.

Switched to allocm() so we always take advantage of all memory that was
actually allocated.

@override-unit-failures
Clearly unrelated
+Warning: This development build of composer is over 30 days old. It
is recommended to update it by running
"hphp/test/slow/ext_phar/composer.php self-update" to get the latest
version.

Test Plan: test added, which failed before and doesn't any more

Reviewed By: lucian@fb.com

FB internal diff: D987009