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/help.json

@@ -27,5 +27,6 @@
     "allc": "allc <network|alias> <(network)|(alias)> <entity> <text ...>",
     "admall": "admall <entity> <text ...>",
     "swho": "swho <network> [<channel>]",
+    "list": "list <network>",
     "exec": "exec <expr ...>"
 }