public Session startCopy2IoBuffer(final String source, final ByteBufferCollector destBuf, final CopyOptions opts) {
        final CopySession session = newCopySession(source);
        session.setOutputStream(null);
        session.setDestBuf(destBuf);
        if (opts != null) {
            session.setCopyOptions(opts);
        }
        session.sendNextRpc();
        return session;
    }
Copy `source` from remote to  buffer.
@param source  source from remote
@param destBuf buffer of dest
@param opt     options of copy
@return true if copy success
jraft-core/src/main/java/com/alipay/sofa/jraft/storage/snapshot/remote/RemoteFileCopier.java