Memory slowly grows over time

[spitfire55]

Because we are lazily reading in the entire JSON log file each time we want to add a new entry, the amount of memory we use also grows with the file.

Instead, Logger.Log(c Connection) should append a new JSON record to the file without reading the entire file in. ioutil.ReadFile(l.fileName) is bad.

Steps to fix:

1. Remove ioutil.ReadFile(l.fileName)
2. For each call to Logger.Log, append the JSON as bytes to the file
3. Make sure to close the file, use mutexes correctly, etc.