Previously, we could lose the heap property when we removed an item
whose parent was greater than the last element in the heap. We would
replace the removed item with the last element, and consider shifting
it down, but we wouldn't consider shifting it up.
Patch from Marko Kreen.
svn:r1227
Backport to 1.4 branch: Use internal implementation for evutil_timercmp() everywhere, to avoid bugs when the platform timercmp() has never heard of <= or >=. Also, replace timercmp() usage in min_heap.c with call to evutil_timercmp().
svn:r745
