message: move leveldb to the bitcoin fork with wider platform support
diff: branch = 63b84335dc590499e5f22498383d6a3432e91ec4
  [submodule "leveldb"]
  path = leveldb
  - url = https://github.com/google/leveldb
  - branch = a53934a3ae1244679f812d998a4f16f2c7f309a6
  + url = https://github.com/bitcoin-core/leveldb
  + branch = bitcoin-fork
  