create dedicated query for bumping workspace activity

[johnstcn]

Each running workspace agent reports stats every 30 seconds by default via POST to /api/v2/workspaceagents/me/report-stats. A side-effect of this operation is that we perform an activity bump which updates the deadline field of the active workspace build.

This curerntly kicks off up to 4 database queries in a transaction:

• GetLatestWorkspaceBuildByWorkspaceID
• GetProvisionerJobByID
• GetWorkspaceByID
• UpdateWorkspaceBuildByID (conditional)

With N active workspaces with active connections phoning home every 30 seconds, this adds potentially N/10 database queries per second.

We can potentially improve this by combining the first three queries into one.