note that a retry is *not* attempted within a given "check readings" run.
rather, server will consider that full readings check to have failed, if any
part of it fails.
but then we keep track of type/amount of some (database connection) failures,
and will suppress logging the full error for first 3 attempts. in practice
this lets us recover from simple database restarts, and if database becomes
truly unavailable we'll hear about it shortly.
any other type of error is immediately logged on first failure.
do not check readings for "archived" clients. do not consider the client
"offline" unless it has *no* current probe readings. previously we were
assuming offline if any probe readings were missing, even if some were found.
