mdbx: release v0.10.3

Acknowledgements:
-----------------
 - [Francisco Vallarino](https://github.com/fjvallarino) for [Haskell bindings for libmdbx](https://hackage.haskell.org/package/libmdbx).
 - [Alex Sharov](https://github.com/AskAlexSharov) for reporting and testing.
 - [Andrea Lanfranchi](https://github.com/AndreaLanfranchi) for contributing.

Extensions and improvements:
----------------------------
 - Added `cursor::erase()` overloads for `key` and for `key-value`.
 - Resolve minor Coverity Scan issues (no fixes but some hint/comment were added).
 - Resolve minor UndefinedBehaviorSanitizer issues (no fixes but some workaround were added).

Fixes:
------
 - Always setup `madvise` while opening DB (fixes https://github.com/erthink/libmdbx/issues/231).
 - Fixed checking legacy `P_DIRTY` flag (`0x10`) for nested/sub-pages.

Minors:
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 - Fixed getting revision number from middle of history during amalgamation (GNU Makefile).
 - Fixed search GCC tools for LTO (CMake scripts).
 - Fixed/reorder dirs list for search CLANG tools for LTO (CMake scripts).
 - Fixed/workarounds for CLANG < 9.x
 - Fixed CMake warning about compatibility with 3.8.2