Introduce more Mission crash failsafes.

[Commiegold]

As a user when a mission experiences a critical error, such as a defense mission becoming stuck with no enemies spawning and no progression possible, I would like the game to recover after a short period of waiting as not to interrupt the user's flow.

See below for an example of a mission becoming stuck.

 A possible solution may include a fail-safe that recognizes that no mission progression has occurred for a prolonged duration, (no new enemies have spawned in x minutes, or the wave duration has hit an unusually long time to complete) and remediates the issue by forcing the next wave to spawn. Alternately, the mission could end in a complete status, allowing the players to acquire the loot drops they'd earned so far on the mission so they can avoid a loss of user progression.