Disable tcache by default if running inside Valgrind, in order to avoid
making unallocated objects appear reachable to Valgrind.

Auto-detect whether running inside Valgrind, thus removing the need to
manually specify MALLOC_CONF=valgrind:true.

Fix prof_realloc() to not call prof_ctx_set() if a sampled object is
being freed via realloc(p, 0).

Refactor code such that arena_mapbits_{large,small}_set() always
preserves the unzeroed flag, and manually manipulate the unzeroed flag
in the one case where it actually gets reset (in arena_chunk_purge()).
This fixes unzeroed preservation bugs in arena_run_split() and
arena_ralloc_large_grow().  These bugs caused large calloc() to return
non-zeroed memory under some circumstances.

Fix the "epoch" mallctl to update cached stats even if the passed in
epoch is 0.

Fix a potential deadlock that could occur during interval- and
growth-triggered heap profile dumps.

Fix an off-by-one heap profile statistics bug that could be observed in
interval- and growth-triggered heap profiles.

Fix heap profile dump filename sequence numbers (regression during
conversion to malloc_snprintf()).

Fix a memory corruption bug in chunk_alloc_dss() that was due to
claiming newly allocated memory is zeroed.

Reverse order of preference between mmap() and sbrk() to prefer mmap().

Clean up management of 'zero' parameter in chunk_alloc*().

Change the "opt.lg_prof_sample" default from 0 to 19 (1 B to 512 KiB).

Change the "opt.prof_accum" default from true to false.

Add the "opt.prof_final" mallctl, so that "opt.prof_prefix" need not be
abused to disable final profile dumping.

Start preparing ChangeLog for 3.0.0 release.  Additional fixes and
changes are yet to come, so this is not a complete ChangeLog.

Fix indentation inconsistencies in ChangeLog.

Add ChangeLog, which briefly summarizes releases.

Edit README and INSTALL.