Document missing upstream changes

[l0rinc]

These were the ones I found here that aren't present in the official repo, many had PRs there already - revived the ones that don't (had to commit under my own name because of their CLA):

• 7045a90
  • Ignore clang's self-assignment check google/leveldb#1261
• 12c52b3
  • win32: fix -Wmissing-field-initializers warnings google/leveldb#1053 - revived in win32: fix -Wmissing-field-initializers warnings google/leveldb#1264
• 1eeb1cb
  • fix macro HAVE_O_CLOEXEC when O_CLOEXEC not found google/leveldb#1028
• f8ae182
  • Adds unicode support to Windows environment. google/leveldb#760
• 92ae82c
  • Increase maximum read-only mmap()s used from 1000 to 4096 on 64-bit systems google/leveldb#613 - revived in Increase maximum read-only mmap()s used from 1000 to 4096 on 64-bit systems google/leveldb#1265
• d42e63d
  • Do not crash if filesystem can't fsync google/leveldb#1262
• bf2c209
  • Add filename to corruption errors google/leveldb#1263

[maflcko]

According to the leveldb readme:

This repository is receiving very limited maintenance. We will only review the following types of changes.

    Fixes for critical bugs, such as data loss or memory corruption
    Changes absolutely needed by internally supported leveldb clients. These typically fix breakage introduced by a language/standard library/OS update

So I am not sure if they will accept stuff like clang-tidy comments in header files that are basically only relevant to bitcoin core

[l0rinc]

Yes, possible, but if we provide PRs maybe they'll merge it and we don't have to keep track of what we have that they don't.