Re: Track the amount of time waiting due to cost_delay

> 46ebdfe164 will interrupt the leaders sleep every time a parallel workers reports
> progress, and we currently don't handle interrupts by restarting the sleep with
> the remaining time. nanosleep does provide the ability to restart with the remaining
> time [1], but I don't think it's worth the effort to ensure more accurate
> vacuum delays for the leader process. 

After discussing offline with Bertrand, it may be better to have 
a solution to deal with the interrupts and allows the sleep to continue to
completion. This will simplify this patch and will be useful 
for other cases in which parallel workers need to send a message
to the leader. This is the thread [1] for that discussion.

[1]
https://www.postgresql.org/message-id/01000190606e3d2a-116ead16-84d2-4449-8d18-5053da66b1f4-000000%40email.amazonses.com

Regards,

Sami