Implement per thread heap profiling.

Rename data structures (prof_thr_cnt_t-->prof_tctx_t,
prof_ctx_t-->prof_gctx_t), and convert to storing a prof_tctx_t for
sampled objects.

Convert PROF_ALLOC_PREP() to prof_alloc_prep(), since precise backtrace
depth within jemalloc functions is no longer an issue (pprof prunes
irrelevant frames).

Implement mallctl's:
- prof.reset implements full sample data reset, and optional change of
  sample interval.
- prof.lg_sample reads the current sample interval (opt.lg_prof_sample
  was the permanent source of truth prior to prof.reset).
- thread.prof.name provides naming capability for threads within heap
  profile dumps.
- thread.prof.active makes it possible to activate/deactivate heap
  profiling for individual threads.

Modify the heap dump files to contain per thread heap profile data.
This change is incompatible with the existing pprof, which will require
enhancements to read and process the enriched data.

include/jemalloc/internal/private_symbols.txt

@@ -48,9 +48,9 @@ arena_prefork
 arena_prof_accum
 arena_prof_accum_impl
 arena_prof_accum_locked
-arena_prof_ctx_get
-arena_prof_ctx_set
 arena_prof_promoted
+arena_prof_tctx_get
+arena_prof_tctx_set
 arena_ptr_small_binind_get
 arena_purge_all
 arena_quarantine_junk_small
@@ -208,8 +208,8 @@ huge_palloc
 huge_postfork_child
 huge_postfork_parent
 huge_prefork
-huge_prof_ctx_get
-huge_prof_ctx_set
+huge_prof_tctx_get
+huge_prof_tctx_set
 huge_ralloc
 huge_ralloc_no_move
 huge_salloc
@@ -287,28 +287,31 @@ opt_zero
 p2rz
 pages_purge
 pow2_ceil
+prof_alloc_prep
 prof_backtrace
 prof_boot0
 prof_boot1
 prof_boot2
 prof_bt_count
-prof_ctx_get
-prof_ctx_set
 prof_dump_open
 prof_free
+prof_free_sampled_object
 prof_gdump
 prof_idump
 prof_interval
 prof_lookup
 prof_malloc
-prof_malloc_record_object
+prof_malloc_sample_object
 prof_mdump
 prof_postfork_child
 prof_postfork_parent
 prof_prefork
 prof_realloc
+prof_reset
 prof_sample_accum_update
 prof_sample_threshold_update
+prof_tctx_get
+prof_tctx_set
 prof_tdata_booted
 prof_tdata_cleanup
 prof_tdata_get
@@ -322,6 +325,10 @@ prof_tdata_tsd_get
 prof_tdata_tsd_get_wrapper
 prof_tdata_tsd_init_head
 prof_tdata_tsd_set
+prof_thread_active_get
+prof_thread_active_set
+prof_thread_name_get
+prof_thread_name_set
 quarantine
 quarantine_alloc_hook
 quarantine_boot