We already ahve raw upgrade loggin, but we need something more user friendly. I am proposing to add a new upgrade_log table which would store important info from upgrade process:
* start date, version before upgrade
* notices/important information
* upgrade failures
* end date, version

On of the first customers would be MDL-16999 - information about changes in Data mod

Implementation:
1/ new upgrade_log table - fields: type, date, string, component, etc...
2/ add upgrade functions -upgrade_log_start(), upgrade_log_info(), upgrade_log_failure() etc.