Implement ChanKeep joining functions

* Low-key channel joining with incrementally increasing delay
* Spin up needed instances to be able to cover a certain channel space
* Fix provisioning functions to prevent race conditions with lots of
relays being created at once
* Tweakable switchover from covering all channels to only covering
channels with more users than the mean of the cumulative user count

conf/example/config.json

@@ -22,6 +22,10 @@
         "User": "sir",
         "Password": "sir"
     },
+    "ChanKeep": {
+        "MaxRelay": 30,
+        "SigSwitch": 20
+    },
     "Dist": {
         "Enabled": true,
         "SendOutput": false,