Squashed 'src/leveldb/' changes from ab6c84e6f3..a7f9bdc611

a7f9bdc611 Merge bitcoin-core/leveldb-subtree#52: Revert "Increase maximum read-only mmap()s used from 1000 to 4096 on 64-bit systems"
a2f531d2d0 Merge bitcoin-core/leveldb-subtree#60: util: use [[fallthrough]] directly
1a166221cf Merge bitcoin-core/leveldb-subtree#61: Disable seek compaction
6bfdb6093b Disable seek compaction
42a5f29aa9 util: use [[fallthrough]] directly
c274b50867 Merge bitcoin-core/leveldb-subtree#53: refactor: Delete unused `ScopedHandle:operator=(ScopedHandle&&)`
68740f586f Merge bitcoin-core/leveldb-subtree#59: Fix Clang `-Wconditional-uninitialized` warning
70142e186b Merge bitcoin-core/leveldb-subtree#55: build: Require C++17
d123cf5a83 Effectively, this change
31361bf339 Allow different C/C++ standards when this is used as a subproject.
0711e6d082 Fix Clang `-Wconditional-uninitialized` warning
85665f9547 refactor: Delete unused `ScopedHandle:operator=(ScopedHandle&&)`
fd8f69657e Revert "Increase maximum read-only mmap()s used from 1000 to 4096 on 64-bit systems"

git-subtree-dir: src/leveldb
git-subtree-split: a7f9bdc6114fe6eeb848fda2980fe61d86ff2045