Prevent multiple execution of commands scheduled in the past

[AngleOSaxon]

Description

• Changes ScheduledMessage.RecalculateStartAfter() logic to advance StartAfter until it moves past the current date
  • A side effect of the implementation is that all StartAfter times are expected to be UTC
• Changes Scheduler.LoadState to call RecalculateStartAfter() on loaded schedules whose StartAfter is before the current time
• Adds hidden parameterless constructor for Logger, to enable some mocking
  • There's probably a better way to do that--I'm not real familiar with PowerShell's testing and mocking libraries

Related Issue

#120

Motivation and Context

If a command was originally scheduled several months ago and the bot is restarted, a very large number of executions will occur very quickly. Depending on the command scheduled, this may prove problematic.

How Has This Been Tested?

Added unit tests to ScheduledMessages.tests.ps1 to check the behavior of RecalculateStartAfter() with dates in the future and the past.

Added unit tests (and some hacky mocks) in Scheduler.tests.ps1 to check the behavior of Scheduler.LoadState() when a schedule with a StartAfter in the past is loaded.

Manually started a bot and ran !newschedule --command 'help' --interval minutes --value 5 --startafter '2018-10-01 12:25pm' and verified that only one response was produced immediately. Restarted the bot and verified that no responses were produced on schedule load.

Screenshots (if appropriate):

Types of changes

• [x ] Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)

Checklist:

• [x ] My code follows the code style of this project.
  • I think? I don't do a lot in PowerShell on a day-to-day basis, so I may have inadvertently missed something.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• [x ] I have read the CONTRIBUTING document.
• [x ] I have added tests to cover my changes.
• [x ] All new and existing tests passed.

[devblackops]

Thanks @AngleOSaxon! 👍