Payroll: remove terminated storage and change exist modifier for activeness check

[sohkai]

cc @lmcorbalan @bingen

Changes the employeeExists modifier to instead check if an employee is still active (their end date hasn't been reached).

This allows the app to:

• Remove the terminated storage for each employee, as invoking terminateEmployeeNow() twice in the same block will fail (employee is only active if their end date hasn't been reached)
• Change details about a yet-to-be "terminated" employee (one who has an non-MAX_UINT64 end date but whose end date has not been reached), such as salary, their actual end date, etc.

Note that this does allow you to "terminate" an employee multiple times, if you set their end date in the future. I'd argue this is a terminology problem though (see below).

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Further potential changes:

• We could change the terminology around "terminating": terminateEmployeeNow() makes sense, but terminateEmployee() should be changed to setEmployeeEndDate() so that its intention is more clear

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Todo:

• Add more tests for terminating employees
• Change terminology to setEmployeeEndDate()

[bingen]

This looks good to me, and I agree with the terminology comments.

[facuspagnuolo]

LGTM as well, lets merge it and I will take care of the tests in another PR

[coveralls]

Coverage remained the same at 96.734% when pulling 2cbff2e on payroll-terminated-employee into 93f0ffb on master.