This will let us unify the startup and runtime-started infrastructure.
Note that there are two kinds of notifications:
1. Starting a single plugin (i.e. `plugin start`)
2. Starting multiple plugins (i.e. `plugin rescan` or `plugin startdir`).
In the latter case, we want the command to complete only once *all*
the plugins are dead/finished.
We also call plugin_kill() in all cases, and correctly return afterwards
(it matters once we use the same paths for dynamic plugins, which don't
cause a fatal error if they don't startup).
Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>
This adds a new pair of files : lightningd/plugin_control, along with a new RPC
command : 'plugin'. This command can be used to manage plugins without restarting lightningd:
lightning-cli plugin start helloworld.py
lightning-cli plugin stop helloworld.py
