Refactored TCPPacketGenerator.

[cying17]

This PR refactors TCPPacketGenerator with adding AppPacketGenerator to send packets from application-level flow.

Description

At the beginning of a simulation, TCPPacketGenerator starts DistPacketGenerator as AppPacketGenerator.

AppPacketGenerator sends packets from application-layer flow to TCPPacketGenerator. TCPPacketGenerator updates self.send_buffer += packet.size after receiving a packet from AppPacketGenerator.

How has this been tested?

python examples/tcp.py

Types of changes

• Bug fix (non-breaking change which fixes an issue) Fixes #
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)

Checklist:

• My code follows the code style of this project.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.