Improvements

• Added a /v2/mbus/status API endpoint that returns metrics about the state of
  the event message bus. This allows for some introspection into what websocket
  connections may be doing at that moment.
• The UI now has less wasted space when displaying a bunch of cards for data
  systems.
• Implemented a configurable timeout when making SSH connections to agents. A
  lower default now also keeps dead agents from taking up large amounts of
  scheduler worker time.
• SHIELD now tracks when the last time an agent erred was.
• Hidden agents are now sorted under a separate header in the web UI.
• Hiding, showing, and deleting agents can now be done from the CLI.

Bug Fixes

• We no longer leak the file descriptors and goroutines for detached websocket
  clients
• Workers can no longer be starved out when sending events to the message bus
  if the receiver of the message bus is misbehaving because these event sends
  are now asynchronous.
• Fixed a bug where a worker could derefence a nil pointer when certain
  database selects returned no rows.
• The database layer now has more stringent locking, which both avoids certain
  threads locking each other out in SQLite, and also makes certain series of
  database operations effectively atomic.
• A couple of fixups would deadlock themselves out of the database and prevent
  fixups from actually running. Now they don't.
• Fixups now only run once instead of on every startup, like nature intended.
• The agent "Last Checked At" timestamp was being updated when the task was
  pulled off the scheduler, whether or not the agent was actually checked
  (due to other potential errors).
• Named a fixup without a name.
• Agents that failed their status checks are now once again marked as such.