Make Design Changes for OrderDeliverer

[vaughnshaun]

Based on a review done by dfhwze of codereview.stackexchange there are some basic code snippets that need to be changed. The full review can be found at https://codereview.stackexchange.com/questions/221757/drone-delivery-scheduling-service/221761#221761. The changes are listed below.

1. Public facing methods should have guard clauses.
2. Keep nested if statements to a minimum.
3. Avoid redundant comments. Only add comments if it adds new information to the code.
4. Inline variable declaration. This is useful for out arguments.
5. Avoid redundant variable type. Use var.
6. Event lifetime management. Events should be unsubscribed.
7. Only use virtual methods if there is a reason