Develop: Add a component to record data usage for every user

[frankfzw]

In order to support multiple users to use the COW proxy without running out of the capacity of VPS by a few, I add a component to limit the usage of every user.

The major addition is usage.go.

• Configuration: A new component to do usage recording. The limitation of every user is set in a file named userCapacityFile (more detail in doc/sample-config/rc). It's loaded when the COW starts. The userPasswdFile must been enabled to use the recording. See more details in doc/sample-config/rc .
• Limitation Check: The limitation check is triggered after Authenticate in proxy.go . Error page is returned if the capacity is run out of.
• Recording: To record the usage, the accumulatedUsage is called for every HTTP response ( doRequest in proxy.go) and every HTTP CONNECT (copyServer2Client in proxy.go). The current usage is first loaded from _cowDir/record.log if existed. A background thread flush the in-memory record to the disk file every half hour. The record will be reset on every reset date (more detail in doc/sample-config/rc).

This time I fixed the previous data racing bug in usage.go