independent flag for json structured logging

[zelig]

re #538

• logjson flag remove logformat flag
• passed to eth Config
• logsystem not a field of Ethereum
• LogSystem does not need to expose GetLogLevel/SetLogLevel
• message struct just implements more generic LogMsg interface
• LogMsg is a fmt.Stringer with Level()
• jsonMsg ([]byte) implements LogMsg
• remove "raw" systems
• move level logic inside StdLogSystem
• logsystems only print their kind of msg: jsonLogSystem prints jsonMsg, StdLogSystem prints stdMsg

[zelig]

re #538 i hacked something together quick,still horribly inefficient
my 2c: if we take format seriously, all logs should be structured originally, which is not realistic. alternatively, we see it from the clumsiness of this code that jsonlogsystem should be completely separate from the other logging, or systemIn channels should be typed similar to event subscription. But most importantly loglevel check should be as global as possible for effieciency, even allowing compile time -nodebug

[tgerring]

+1 tested locally and works as expected

ethereum --loglevel "3" to output Info logs and above (Default setting)
ethereum --logjson "-" --loglevel "0" to output JSON-only logs
ethereum --logjson "-" --loglevel "5" to output JSON in addition to DebugDetail (and above) logs

[tgerring]

Tagging @LefterisJP @sveneh @heikoheiko to let them know of this pending change