Fix issue with RPC delivery when R is busy
Fix issue with RPC delivery when R is busy

This resolves #1970, resolves #1957 and backs-out 2a131ea9.
JanMarvin/rstudio,JanMarvin/rstudio,JanMarvin/rstudio,JanMarvin/rstudio,JanMarvin/rstudio,JanMarvin/rstudio,JanMarvin/rstudio,JanMarvin/rstudio,JanMarvin/rstudio