0.12.1 "Positive Proxima"

The planned frontward release with new superior features on the day of 20 anniversary of [Positive Technologies](https://ptsecurty.com).

New:
---

 - The `Big Foot` feature which significantly reduces GC overhead for processing large lists of retired pages from huge transactions.
   Now _libmdbx_ avoid creating large chunks of PNLs (page number lists) which required a long sequences of free pages, aka large/overflow pages.
   Thus avoiding searching, allocating and storing such sequences inside GC.
 - Improved hot/online validation and checking of database pages both for more robustness and performance.
 - New solid and fast method to latch meta-pages called `Troika`.
   The minimum of memory barriers, reads, comparisons and conditional transitions are used.
 - New `MDBX_VALIDATION` environment options to extra validation of DB structure and pages content for carefully/safe handling damaged or untrusted DB.
 - Accelerated ×16/×8/×4 by AVX512/AVX2/SSE2/Neon implementations of search page sequences.
 - Added the `gcrtime_seconds16dot16` counter to the "Page Operation Statistics" that accumulates time spent for GC searching and reclaiming.
 - Copy-with-compactification now clears/zeroes unused gaps inside database pages.
 - The `C` and `C++` APIs has been extended and/or refined to simplify using `wchar_t` pathnames.
   On Windows the `mdbx_env_openW()`, `mdbx_env_get_pathW()`, `mdbx_env_copyW()`, `mdbx_env_open_for_recoveryW()` are available for now,
   but the `mdbx_env_get_path()` has been replaced in favor of `mdbx_env_get_pathW()`.
 - Added explicit error message for Buildroot's Microblaze toolchain maintainers.
 - Added `MDBX_MANAGE_BUILD_FLAGS` build options for CMake.
 - Speed-up internal `bsearch`/`lower_bound` implementation using branchless tactic, including workaround for CLANG x86 optimiser bug.
 - A lot internal refinement and micro-optimisations.
 - Internally counted volume of dirty pages (unused for now but for coming features).

Fixes:
-----

 - Never use modern `__cxa_thread_atexit()` on Apple's OSes.
 - Don't check owner for finished transactions.
 - Fixed typo in `MDBX_EINVAL` which breaks MingGW builds with CLANG.

For earlier versions please refer to the git's tags and the [ChangeLog](https://gitflic.ru/project/erthink/libmdbx/blob?file=ChangeLog.md).

37 files changed, 7604 insertions(+), 7417 deletions(-)
Signed-off-by: Леонид Юрьев (Leonid Yuriev) <leo@yuriev.ru>
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