v5.6.1

Main Changes to previous release version:

• Support to disabling offline message.
• Resolve internal memory leakage issues.
• Fix curl dependencies.
• update libressl to v2.9.2.
• Update Dockerfile to ubuntu:18.04 and correct docs variable reference.
• update flatcc to 0.6.0.
• Fix connector destroy bug.
• Support pre-defined secret key.
• fix send_friend_message side issue.
• Improving express node access order.
• Fix ios static library install issue.
• Fix receipt thread sync issue.