Merge #13925: Merge leveldb subtree

ec749b1 Squashed 'src/leveldb/' changes from 64052c76c5..524b7e36a8 (MarcoFalke)

Pull request description:

  For review:

  ```sh
  git fetch https://github.com/bitcoin-core/leveldb
  ./test/lint/git-subtree-check.sh src/leveldb
  ```

  Closes #13860

Tree-SHA512: 9d13384fe35e7144b4a7fca57efe77b0cc5295952da4a397e4c6d8aa3f8043d5113fccedd3ae1dcaa3d2649e732e5f57a71504847946e055aa4dc8c3780e29fc

Author: laanwj

src/leveldb/util/env_posix.cc

@@ -585,8 +585,8 @@ static int MaxMmaps() {
   if (mmap_limit >= 0) {
     return mmap_limit;
   }
-  // Up to 1000 mmaps for 64-bit binaries; none for smaller pointer sizes.
-  mmap_limit = sizeof(void*) >= 8 ? 1000 : 0;
+  // Up to 4096 mmaps for 64-bit binaries; none for smaller pointer sizes.
+  mmap_limit = sizeof(void*) >= 8 ? 4096 : 0;
   return mmap_limit;
 }