Aesop Patch to send old values from mysql producer

[akshit-agarwal]

These changes are to enable old value propagation from mysql relay (producer). This is a flag based change, whereby in sample-mysql-relay this flag can be set. When enabled, in DBChangeEntry generic record which use to carry mysql-row field information (updated) will now have extra key: '_oldValue' (which will be a hash containing old values of changed field). The '_oldValue' column name depends on avro-schema defined. In schema-generator utility, a command line parameter is added which can be used if someone wants to generate schema with '_oldValue' column (column name is user defined in parameter). This will update meta properties in avro-schema, mentioning '_oldValue' is special column (not mysql column). At clients side, in AbstractEvent a new field stores the information of '_oldValue' from DataBus event and can be used if required.

@jagadeesh-huliyar please have a look