XDCR ns_server Integration -- xdcr logging

ns_server needs to provide mechanism to write xdcr logs to log files.

The current plan is that:
xdcr writes all logs to stdout. ns_server writes these logs to separate log files in accordance with the log levels, and does log file rotation when needed.
– xdcr.log for info logs
– xdcr_error.log for error logs
– xdcr_trace.log for debug and trace logs