Remove hpa_sec_batch_fill_extra and calculate nallocs automatically.

This change includes the following improvements:

- Remove the hpa_sec_batch_fill_extra parameter.
- Refactor the hpa_alloc() code and helper functions to be able to
  allocate more than one extent out of a single pageslab. This way
  we can amortize the per-pageslab costs (active bitmap iteration,
  pageslab metadata updates) across multiple extents.
- Decide on a min and max number of extents that will be allocated
  in hpa_alloc(). The code will try to allocate at least the min
  and allocate up to the max as long as we can allocate additional
  ones from the pageslab we already have, as additional allocations
  are relatively cheap.
- Add extent allocation distribution stats.
- Amend hpa_sec_integration.c unit test.

src/jemalloc.c

@@ -162,13 +162,13 @@ void (*JET_MUTABLE junk_free_callback)(
     void *ptr, size_t size) = &default_junk_free;
 void (*JET_MUTABLE invalid_conf_abort)(void) = &abort;
 
-bool         opt_utrace = false;
-bool         opt_xmalloc = false;
-bool         opt_experimental_infallible_new = false;
-bool         opt_experimental_tcache_gc = true;
-bool         opt_zero = false;
-unsigned     opt_narenas = 0;
-fxp_t opt_narenas_ratio = FXP_INIT_INT(4);
+bool     opt_utrace = false;
+bool     opt_xmalloc = false;
+bool     opt_experimental_infallible_new = false;
+bool     opt_experimental_tcache_gc = true;
+bool     opt_zero = false;
+unsigned opt_narenas = 0;
+fxp_t    opt_narenas_ratio = FXP_INIT_INT(4);
 
 unsigned ncpus;
 
@@ -208,7 +208,6 @@ typedef struct {
 #	define UTRACE(a, b, c)
 #endif
 
-
 /******************************************************************************/
 /*
  * Begin miscellaneous support functions.