Refactor purging and splitting/merging.

Split purging into lazy and forced variants.  Use the forced variant for
zeroing dss.

Add support for NULL function pointers as an opt-out mechanism for the
dalloc, commit, decommit, purge_lazy, purge_forced, split, and merge
fields of extent_hooks_t.

Add short-circuiting checks in large_ralloc_no_move_{shrink,expand}() so
that no attempt is made if splitting/merging is not supported.

This resolves #268.

src/large.c

@@ -110,6 +110,9 @@ large_ralloc_no_move_shrink(tsdn_t *tsdn, extent_t *extent, size_t usize)
 
 	assert(oldusize > usize);
 
+	if (extent_hooks->split == NULL)
+		return (true);
+
 	/* Split excess pages. */
 	if (diff != 0) {
 		extent_t *trail = extent_split_wrapper(tsdn, arena,
@@ -142,6 +145,9 @@ large_ralloc_no_move_expand(tsdn_t *tsdn, extent_t *extent, size_t usize,
 	size_t trailsize = usize - extent_usize_get(extent);
 	extent_t *trail;
 
+	if (extent_hooks->merge == NULL)
+		return (true);
+
 	if ((trail = arena_extent_cache_alloc(tsdn, arena, &extent_hooks,
 	    extent_past_get(extent), trailsize, CACHELINE, &is_zeroed_trail)) ==
 	    NULL) {