update the LoggerGetLogContentHandler to return map and handle parsing exception

[stevehu]

1. There might be multiple loggers defined in the logback.xml with different logging levels. The UI will have a dropdown to displace the content for a logger each time.
2. Add the offset and limit to the API so that only a number of the logging statements will be returned for each request.
3. Handle the exception to parse the logging statement to skip logger like Audit etc.
4. Change the return to a map of maps with logger as key and total and logs as key for the value to support pagination.
5. Filter the record by level and logger name when adding the line to the logs.