Implement dynamic per arena control over dirty page purging.

Add mallctls:
- arenas.lg_dirty_mult is initialized via opt.lg_dirty_mult, and can be
  modified to change the initial lg_dirty_mult setting for newly created
  arenas.
- arena.<i>.lg_dirty_mult controls an individual arena's dirty page
  purging threshold, and synchronously triggers any purging that may be
  necessary to maintain the constraint.
- arena.<i>.chunk.purge allows the per arena dirty page purging function
  to be replaced.

This resolves #93.

include/jemalloc/internal/private_symbols.txt

@@ -30,6 +30,10 @@ arena_dalloc_small
 arena_dss_prec_get
 arena_dss_prec_set
 arena_init
+arena_lg_dirty_mult_default_get
+arena_lg_dirty_mult_default_set
+arena_lg_dirty_mult_get
+arena_lg_dirty_mult_set
 arena_malloc
 arena_malloc_large
 arena_malloc_small
@@ -151,6 +155,9 @@ chunk_npages
 chunk_postfork_child
 chunk_postfork_parent
 chunk_prefork
+chunk_purge_arena
+chunk_purge_default
+chunk_purge_wrapper
 chunk_record
 chunk_register
 chunks_rtree