rename nginx parser to server_log parser
Move payload decoding logic out of server_log parser
server_log changes

 - No longer refer to NginxLogParser
 - make serverLogParser non public
 - change MakeErrorEvent() signature to take primitives for uuid and
   timestamp as json_log parser is all or nothing - unlike the
   server_log parser which is a best attempt parser
Bugfix: do not pass ints to json.Number
Json Parser Implementation