WorkTasks: destruct data before invoking callback

[kkaefer]

This is a short term fix for #3636. The reason for the hangs we were seeing was because the following happened:

• Work task completes, calls callback, which schedules the after callback on the main thread
• After callback was invoked and tried to cancel the work task first thing, which blocks on the mutex
• On the worker thread, we were destructing the arguments (in particular the VectorTile object) after we've called the callback in the first step; on slow devices, this could take 20-100ms
• Worker thread releases mutex, then main thread can finally acquire it to mark the task as canceled

This patch fixes this with the following changes:

• Move work task arguments from the tuple into the actual function, so they can be destructed there before the callback gets fired
• Actually destruct the arguments by either resetting, or moving the data into the invocation where it will get destructed

[kkaefer]

by moving the entire unique_ptr, we ensure that it gets destructed when this function returns.

[kkaefer]

With this fix, we almost never wait for WorkTask cancelations (= immediate lock acquisition), and if we do, it's only for a very short period of time.

[jfirebaugh]

👍