Migrating from 8.10 to 9.3-SNAP Audit is not working

moving ES audit from 8.10 to 9.3-SNAP is not working on write:

2017-10-30 17:32:08,183 ERROR [AuditLogWriterPool-00,in:11,inCheckpoint:10,out:0,lastRead:1509381128023,lastTimer:1509381127972,wm:1,loop:2576,record] [org.nuxeo.elasticsearch.audit.ESAuditBackend] Unable to index audit entry 0 :MapperParsingException[failed to parse [eventDate]]; nested: IllegalArgumentException[Invalid format: "1509381128017" is malformed at "8017"];

This is because the 8.10 mapping expect dateOptionalTime format and the new logEntry json serializer (which is now a jackson mapper instead of the adhoc AuditEntryJSONWriter) represents date as millisecond timestamp.

The log entry json representation send to ES must be updated to send dateOptionalTime compatible date so we can keep the 8.10 mapping and don't need a migration procedure.