when $metrics.lastexecutionstart is null, set it to 0

[dengshilong]

When $metrics.lastexecutionstart is null, set it to 0, so we can do a full-import. This is useful when select incremental data from a table.

[jprante]

Not sure if this sets timestamp to just 01-JAN-1970, but I accept it for now... challenges with date/times before 1970 are another subject that matters throughout the importer code.

[jprante]

Thanks!

[dengshilong]

I have not consider the time before 01-JAN-1970. Now we don't need

There is a problem here, the first time you run the script, it can't select any data from table, it have two solutions here:

in another script, do full-import, later you can use the incremental script to select incremental data
define a statefile.json file before the first time you run the incremental script, set the lastexecutionstart to 0, so that you can select all the data from table.

I will delete it from README.