tim-connector: Add protected default constructor for Hibernate to new entities

Include proper comments warning about not using it directly.

FEA: ItEr77S16JiraAndTimConnectorContributionIntegration
tim-connector: Use LocalDate API

It makes code simpler than using Calendar API, and it's the common way to manage
dates into LibrePlan.

FEA: ItEr77S16JiraAndTimConnectorContributionIntegration
tim-connector: Remove commented line

FEA: ItEr77S16JiraAndTimConnectorContributionIntegration
tim-connector: Avoid to load all workers to import/export timesheets from/to Tim

Maybe it's not needed to get all workers, so the call to findAll has been changed by findByCode.

Moreover, it was using code in ExportTimesheetsToTim and NIF in
ImportRosterFromTim which was not coherent, so it has been changed to use code
in both places.

FEA: ItEr77S16JiraAndTimConnectorContributionIntegration