BookInfoManager: Minor fixes & tweaks in `collectSubprocesses`

* Don't run multiple collectSubprocesses tasks in parallel.
  UIManager:scheduleIn doesn't return anything, using that as a boolean trap made no sense.

* Make the in-place removal of collected pids use a slightly more common idiom
  (table.remove in a reverse iteration of said array).

plugins/coverbrowser.koplugin/bookinfomanager.lua

@@ -617,25 +617,27 @@ end
 
 -- Background extraction management
 function BookInfoManager:collectSubprocesses()
+    self.subprocesses_collector = nil
+
     -- We need to regularly watch if a sub-process has terminated by
     -- calling waitpid() so this process does not become a zombie hanging
     -- around till we exit.
     if #self.subprocesses_pids > 0 then
-        local i = 1
-        while i <= #self.subprocesses_pids do -- clean in-place
+        -- In-place removal, hence the reverse iteration.
+        for i = #self.subprocesses_pids, 1, -1 do
             local pid = self.subprocesses_pids[i]
             if FFIUtil.isSubProcessDone(pid) then
                 table.remove(self.subprocesses_pids, i)
                 -- Prevent has been issued for each bg task spawn, we must allow for each death too.
                 UIManager:allowStandby()
-            else
-                i = i + 1
             end
         end
         if #self.subprocesses_pids > 0 then
             -- still some pids around, we'll need to collect again
-            self.subprocesses_collector = UIManager:scheduleIn(
-                self.subprocesses_collect_interval, function()
+            self.subprocesses_collector = true
+            UIManager:scheduleIn(
+                self.subprocesses_collect_interval,
+                function()
                     self:collectSubprocesses()
                 end
             )
@@ -650,7 +652,6 @@ function BookInfoManager:collectSubprocesses()
                 -- we'll collect them next time we're run
             end
         else
-            self.subprocesses_collector = nil
             if self.delayed_cleanup then
                 self.delayed_cleanup = false
                 -- No more subprocesses = no more crengine indexing, we can remove our
@@ -661,7 +662,9 @@ function BookInfoManager:collectSubprocesses()
     end
 
     -- We're done, back to a single core
-    Device:enableCPUCores(1)
+    if #self.subprocesses_pids == 0 then
+        Device:enableCPUCores(1)
+    end
 end
 
 function BookInfoManager:terminateBackgroundJobs()
@@ -722,8 +725,10 @@ function BookInfoManager:extractInBackground(files)
     -- and fill linux processes table)
     -- We set a single scheduled action for that
     if not self.subprocesses_collector then -- there's not one already scheduled
-        self.subprocesses_collector = UIManager:scheduleIn(
-            self.subprocesses_collect_interval, function()
+        self.subprocesses_collector = true
+        UIManager:scheduleIn(
+            self.subprocesses_collect_interval,
+            function()
                 self:collectSubprocesses()
             end
         )