TCP review - adds comments, tests, and reorganizes some methods

[howardwu]

Motivation

This PR adds and updates comments and traces in the Tcp stack, and reorganizes methods for easier readability.

The following are the only logic changes to the Tcp stack.

• Abstracts the listening task logic into 2 new methods: enable_listener and handle_connection
• Adds Tcp::connecting_addrs to retrieve the pending IP addresses
• (https://github.com/AleoHQ/snarkOS/pull/2143/commits/316c75654a2753857aeb747bb065ad150c497d46) Switches !can_add_connection error from PermissionDenied to ConnectionRefused

Test Plan

This PR adds tests on the following Tcp methods:

• new
• connect
• disconnect
• can_add_connection
• handle_connection
• adapt_stream

Related PRs

None

[howardwu]

@ljedrz I was careful to ensure no logic changes took place besides the 3 bullet points enumerated in this PR description.