Use Avro Long epoch for representing XML dateTime

Date/Times are often present in XML data (time-stamps) and are 
often taking a lot of CPU for parsing and bytes for storing them
as strings.

Using an Avro Long type for their corresponding Unix Epoch mills
would allow to have a faster processing and more compact storage.
Merge branch 'datetime-to-epoch' of https://github.com/lucamilanesio/xml-avro into lucamilanesio-datetime-to-epoch
Merge branch 'lucamilanesio-datetime-to-epoch'
misc changes about datetime support