lib/config, lib/model: Limit concurrent pulls (fixes #5914) (#6290)

Adds a new folder state "Waiting to Sync" in the same vein as the
existing "Waiting to Scan". This vastly improves performances in the
rare cases when there are lots and lots of folders operating.

test/folders.sh

@@ -0,0 +1,11 @@
+#!/bin/bash
+
+for ((id=0;id<200;id++)); do
+cat <<EOT
+    <folder id="$id" label="" path="$id?maxsize=1000&amp;seed=$id" type="sendreceive" rescanIntervalS="3600" fsWatcherEnabled="false" fsWatcherDelayS="10" ignorePerms="false" autoNormalize="true">
+        <filesystemType>fake</filesystemType>
+        <device id="I6KAH76-66SLLLB-5PFXSOA-UFJCDZC-YAOMLEK-CP2GB32-BV5RQST-3PSROAU" introducedBy=""></device>
+        <device id="MRIW7OK-NETT3M4-N6SBWME-N25O76W-YJKVXPH-FUMQJ3S-P57B74J-GBITBAC" introducedBy=""></device>
+    </folder>
+EOT
+done