message: refactor: get wallet path relative to wallet_dir
  Now that boost has been updated > 1.60, we can simplify how we get
  wallet path relative to wallet_dir by using:
  `boost::filesystem::lexically_relative`
diff: std::vector<fs::path> ListDatabases(const fs::path& wallet_dir)
  {
  - const size_t offset = wallet_dir.native().size() + (wallet_dir == wallet_dir.root_name() ? 0 : 1);
  std::vector<fs::path> paths;
  boost::system::error_code ec;
  @@ -28,10 +27,7 @@ std::vector<fs::path> ListDatabases(const fs::path& wallet_dir)
  }
  try {
  - // Get wallet path relative to walletdir by removing walletdir from the wallet path.
  - // This can be replaced by boost::filesystem::lexically_relative once boost is bumped to 1.60.
  - const auto path_str = it->path().native().substr(offset);
  - const fs::path path{path_str.begin(), path_str.end()};
  + const fs::path path{it->path().lexically_relative(wallet_dir)};
  if (it->status().type() == fs::directory_file &&
  (IsBDBFile(BDBDataFile(it->path())) || IsSQLiteFile(SQLiteDataFile(it->path())))) {
  