Fixes for Issues with Beacon Tasks

[RafBishopFox]

I received a report that beacon tasks could not be cancelled because of a database issue. It turns out that listing beacon tasks was not possible because of a related root issue.

When cancelling a beacon task, saving the object after updating the state field caused GORM to write the timestamp for the created_at column as an integer. When the created_at column was read back in, GORM was trying to convert the integer value into a time.Time object which it did not like. Since created_at is only modified at creation time, there is no need to update the entire object using Save(). This PR updates only the State field when a task is cancelled which avoids messing with the created_at field.

When listing beacon tasks, something similar was happening. When we were selecting tasks using .Select() to avoid grabbing the request and response columns, that bypasses GORM's field to column mapping, and GORM tries to cast the timestamp into a time.Time object unsuccessfully. This PR grabs the entire object and instructs the ToProtobuf function to omit the Request and Response fields.

While I was going down these rabbit holes, I noticed that if you cancelled a task and specified its ID (tasks cancel [ID]), the server panicked. It looks like the Request column was pulled and when passed back to the server, it panicked. This PR nils those fields before sending the task to the server.