[$60] Replace Job.endDate with Job.duration

[maxceem]

After clarifying business requirements we find out that we don't need such a complicated logic as was suggested in #113. We would like to make the next update:

• Remove Job.endDate field
• Add a new optional Job.duration field. It should only accept integer values 1 or more
• Update Swagger and Postman
• Update ES mapping config in API
• Update ES Processor
• Create a migration script. No need to migrate existent data, i. e. it's fine to have duration=null for all existent records.

[maxceem]

Challenge https://www.topcoder.com/challenges/f2e057d5-f206-4d0c-86eb-53031c90a4ed has been created for this ticket.

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[maxceem]

@imcaizheng this is ready for pickup.

[maxceem]

Challenge https://www.topcoder.com/challenges/f2e057d5-f206-4d0c-86eb-53031c90a4ed has been assigned to aaron2017.

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[imcaizheng]

PR created #145 and topcoder-platform/taas-es-processor#25