Allow BASE_BACKUP to be throttled
 
A new MAX_RATE option allows imposing a limit to the network transfer
rate from the server side.  This is useful to limit the stress that
taking a base backup has on the server.
 
pg_basebackup is now able to specify a value to the server, too.
 
Author: Antonin Houska
 
Patch reviewed by Stefan Radomski, Andres Freund, Zoltán Böszörményi,
Fujii Masao, and Álvaro Herrera.