Document logging file size cap for esp and rolling

The size of ESP log files may be limited, so that if the log files is likely to (or has already exceeded) the preset limit, the log files are rolled over.

By default size-based limit is disabled.

The size limit is provided in ESP's configuration in the Attributes tab. The field is named: maxLogFileSize.  If set to '0', size limits are disabled.

The change has required esp log file names to change a little.  Previously, esp log file names were in the form esp_main.YYYY_MM_DD.log.  However, if log files are rolled over as a result of the ESP log file being too big, the new log filename will be in the form: esp_main.YYYY_MM_DD_HH_MM_SS.log.