limit rate of webxdc updates (#3417)

* more flexible render_webxdc_status_update_object()

* delay webxdc updates when ratelimit is reached

* inject updates messages to the SMTP loop as needed

this avoids starting several tasks
and allows sending updates out after a restart of the app.

* use mutex to prevent race conditions in status updates

* check ratelimiter only before the sending loop; it won't change until messages are actually sent out

* fix typo

* prefer standard type declaration over turbofish syntax

* use UNIQUE and ON CONFLICT for query smtp_status_updates

* combine DELETE+SELECT to one atomic statement

* as all operations on smtp_status_updates are now atomic, a mutex is no longer needed

* test DELETE+RETURNING statement

* simplify calls to can_send()

* comment about ratelimit boolean in send_smtp_messages()

CHANGELOG.md

@@ -9,6 +9,7 @@
 - format message lines starting with `>` as quotes #3434
 - node: remove `split2` dependency #3418
 - node: add git installation info to readme #3418
+- limit the rate of webxdc update sending #3417
 
 ### Fixes
 - set a default error if NDN does not provide an error