Add better documentation for the TGen state machine and timings

[robgjansen]

TGen reports transfer successes and failures in the log files using the stream-success and stream-failure keywords. These log messages report several timestamps that we use to compute download time and other metrics (e.g., circuit build times when downloading through Tor).

We should better document the steps of the state machine and clearly describe the times.

Here is a quick dump. From the TGen client's perspective, here is the transfer process when using Tor:

Step	Log file timestamp key	Description
1.	usecs-to-socket-create	Open's socket
2.	usecs-to-socket-connect	Connect to Tor’s socks server port
3.	usecs-to-proxy-init	Client initiates SOCKS handshake
4.	usecs-to-proxy-choice	SOCKS server responds to handshake
5.	usecs-to-proxy-request	Tell Tor to open connection to a tgen server
6.	usecs-to-proxy-response	Tor says OK connection opened
7.	usecs-to-command	TGen client sends (through Tor) command to tell the TGen server what to do
8.	usecs-to-response	Client received initial TGen server response to command
A9.	usecs-to-first-byte-send	Client sent first byte of transfer
A10.	usecs-to-last-byte-send	Client sent last byte of transfer
A11.	usecs-to-checksum-send	Client sent checksum computed over sent data
B9.	usecs-to-first-byte-recv	Client received first byte of transfer from server
B10.	usecs-to-last-byte-recv	Client received last byte of transfer from server
B11.	usecs-to-checksum-recv	Client received server's checksum computed over received data

The "A" and "B" phases happen concurrently as the client sends data to and receives data from the server, respectively.

The "transfer" time starts when the command is sent (step 7), and ends when checksums are both sent and received (after steps A11 and B11).