Revised the implementation of RTO.

[cying17]

This PR revises the implementation of RTO.

Description

This PR revises the implementation of RTO in Timer. Each packet's Timer has its own RTO, which is initializes to self.rto of TCPPacketGenerator and will be doubled every time the packet is retransmitted.

How has this been tested?

python examples/fattree_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.