[BUGZILLA #17940] Running URL checks concurrently

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#### METADATA

 - Bug author - Jim Hester
 - Creation time - 2020-10-05 20:43:30 UTC
 - [Bugzilla link](https://bugs.r-project.org/bugzilla/show_bug.cgi?id=17940)
 - Status - UNCONFIRMED
 - Alias - None
 - Component - Misc
 - Version - R-devel (trunk)
 - Hardware - Other Other
 - Importance - P5 normal
 - Assignee - R-core
 - URL - 
