disconnected clients: Add reconnect task event

[DerekStrickland]

This PR introduces a new TaskClientReconnected task event. This task event will be appended by the allocRunner each time an unknown alloc is reconnected after the client comes back online.

This PR includes:

• The changes related to defining the TaskEvent and translating it to meaningful human text for display.
• Refactors the reconnect logic out of the client and into the allocRunner as it has grown in scope. Specifically, the logic workflow is now:
  • Appends a task event entry for allocs still running on the client.
  • Updates the existing alloc instance with the indexes from the incoming update.
  • Recomputes task states with new events.
  • Calls clientAlloc to compute the current alloc state including the appended events.
  • Updates the client state store with the updated alloc. Exits if this fails.
  • Propagates the client alloc state to the server.
  • Broadcasts the alloc update to all listeners.
  • Removes reconnects from the Plan as they will be pulled by the node when it reconnects.

[DerekStrickland]

Follow up question in thread

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