installinator: progress reports are intermittent when there are unresponsive bootstrap peers

#3159 fixed an issue that would cause installinator downloads to time out if it tried to contact unresponsive peers on the bootstrap network; however, it revealed a less serious issue: when sending any given progress report, installinator waits until it has finished sending to all peers before potentially sending another report. If there are unreponsive peers, that may take ~15 seconds (our current HTTP connection timeout), which means peers that are responsive (and in particular, the one peer that really cares about progress) only sees a new report every ~15 seconds.

This doesn't prevent mupdate from running but does make the installinator section of it feel sluggish / unresponsive. We should fix if we can but it's not a priority above missing functionality.

Trace installinator's progress-report sending path and its HTTP connection timeout, using the issue's peer-send behavior as the starting point. Verify completion by confirming responsive peers receive reports without waiting roughly 15 seconds on unresponsive peers, and that mupdate still runs.