[feat] Implement Close()

[kinbiko]

This method flushes the unsent reports and sessions upon being invoked. Note: This method does not conform to the io.Closer interface because it doesn't return an error. I'm not a fan of this function returning an error in the first place, so instead of propagating unnecessary defer func(){_ = n.Close()}() where the Close func never returns an error anyway just to adhere to the interface, I decided to not return anything in the first place.

Additional changes:

• Conform to a stricter "gofumports" formatting ruleset
• Define a larger size for the buffered channels since both sessions and error reports share the same select statement.
• Stop flushing sessions in new goroutines, and get rid of the mutexes.

[kinbiko]

Need to update the docs on New to indicate that n.Close() should also be called when closing the app. Needs testing.