_rcopy(srcPath, dstPath) {
    return new Promise((resolve, reject) => {
      const ws = this.sftp.createWriteStream(dstPath);
      const rs = this.sftp.createReadStream(srcPath);
      ws.on('error', (err) => {
        reject(this.fmtError(`${err.message} ${dstPath}`, '_rcopy'));
      });
      rs.on('error', (err) => {
        reject(this.fmtError(`${err.message} ${srcPath}`, '_rcopy'));
      });
      ws.on('close', () => {
        resolve(`${srcPath} copied to ${dstPath}`);
      });
      rs.pipe(ws);
    });
  }
@async

Make a remote copy of a remote file. Create a copy of a remote file on the remote
server. It is assumed the directory where the copy will be placed already exists.
The destination file must not already exist.

@param {String} srcPath - path to the remote file to be copied
@param {String} dstPath - destination path for the copy.

@returns {String}.