Introduce pinned extents to contain unpurgeable pages

Some pages (e.g., hugetlb pages) cannot be purged, and should be
prioritized for reuse.  A custom extent_alloc hook signals this by
OR'ing EXTENT_ALLOC_FLAG_PINNED into the low bits of the returned
pointer; jemalloc strips the flag bits and caches pinned extents in
a dedicated ecache_pinned, separate from the dirty/muzzy decay
pipeline.

Pinned extents do not coalesce eagerly, except for ones larger than
SC_LARGE_MINCLASS.  A prefer-small policy reuses the smallest fitting
pinned extent, to avoid unnecessary split/fragmentation.

configure.ac

@@ -2062,6 +2062,9 @@ if test "x${LG_PAGE}" != "xundefined" ; then
 else
    AC_MSG_ERROR([cannot determine value for LG_PAGE])
 fi
+if test "${LG_PAGE}" -lt 8 ; then
+   AC_MSG_ERROR([LG_PAGE must be at least 8 (PAGE >= 256)])
+fi
 
 AC_ARG_WITH([lg_hugepage],
   [AS_HELP_STRING([--with-lg-hugepage=<lg-hugepage>],