Avoid deep recursion in appservice recovery (#5885)

Hopefully, this will fix a stack overflow when recovering an appservice.

The recursion here leads to a huge chain of deferred callbacks, which then
overflows the stack when the chain completes. `inlineCallbacks` makes a better
job of this if we use iteration instead.

Clean up the code a bit too, while we're there.

1 files changed, 1 insertions, 0 deletions

diff --git a/changelog.d/5885.bugfix b/changelog.d/5885.bugfix
new file mode 100644
index 0000000000..411d925fd4
--- /dev/null
+++ b/changelog.d/5885.bugfix
@@ -0,0 +1 @@
+Fix stack overflow when recovering an appservice which had an outage.