Changes gRD scheduling to be based on estimated ready time of segments.
Changes gRD scheduling to be based on estimated ready time of segments.

Instead of requesting segments essentially round-robin from backups
recovery masters ask for segments in the order that they expect
backups will have them ready.  This means backups with faster disks
will get more requests for each request from those with slower disks.

This patch seems to tighten recovery time down to the max disk reading
time across the board.  It also seems to interact reasonably with
other segment placement strategies.

behnamm/cs244b_project,utah-scs/RAMCloud,SMatsushi/RAMCloud,taschik/ramcloud,IMCG/RamCloud,y-higuchi/ramcloud,jblomer/ramcloud,SMatsushi/RAMCloud,rstutsman/RAMCloud,IMCG/RamCloud,mrdiegoa/ramcloud,alexandermerritt/ramcloud,DavidLi2010/ramcloud,anirajk/RAMCloud,DavidLi2010/ramcloud,matrix207/RAMCloud,mrdiegoa/ramcloud,y-higuchi/ramcloud,Frank-Wu/RamCloud,rstutsman/RAMCloud,taschik/ramcloud,behnamm/cs244b_project,anirajk/RAMCloud,y-higuchi/ramcloud,jcarreira/ramcloud,anirajk/RAMCloud,SMatsushi/RAMCloud,Frank-Wu/RamCloud,DavidLi2010/ramcloud,IMCG/RamCloud,SMatsushi/RAMCloud,alexandermerritt/ramcloud,alexandermerritt/ramcloud,utah-scs/RAMCloud,jblomer/ramcloud,taschik/ramcloud-load-manager,rstutsman/RAMCloud,taschik/ramcloud,matrix207/RAMCloud,taschik/ramcloud,QingkaiLu/RAMCloud,mrdiegoa/ramcloud,SMatsushi/RAMCloud,anirajk/RAMCloud,alexandermerritt/ramcloud,utah-scs/RAMCloud,y-higuchi/ramcloud,behnamm/cs244b_project,IMCG/RamCloud,QingkaiLu/RAMCloud,mrdiegoa/ramcloud,taschik/ramcloud-load-manager,mrdiegoa/ramcloud,jblomer/ramcloud,jblomer/ramcloud,QingkaiLu/RAMCloud,anirajk/RAMCloud,utah-scs/RAMCloud,Frank-Wu/RamCloud,IMCG/RamCloud,alexandermerritt/ramcloud,QingkaiLu/RAMCloud,utah-scs/RAMCloud,taschik/ramcloud,y-higuchi/ramcloud,DavidLi2010/ramcloud,matrix207/RAMCloud,rstutsman/RAMCloud,jcarreira/ramcloud,jblomer/ramcloud,y-higuchi/ramcloud,utah-scs/RAMCloud,matrix207/RAMCloud,Frank-Wu/RamCloud,QingkaiLu/RAMCloud,taschik/ramcloud-load-manager,taschik/ramcloud-load-manager,matrix207/RAMCloud,mrdiegoa/ramcloud,behnamm/cs244b_project,jblomer/ramcloud,Frank-Wu/RamCloud,matrix207/RAMCloud,anirajk/RAMCloud,taschik/ramcloud-load-manager,rstutsman/RAMCloud,behnamm/cs244b_project,SMatsushi/RAMCloud,rstutsman/RAMCloud,jcarreira/ramcloud,jcarreira/ramcloud,jcarreira/ramcloud,DavidLi2010/ramcloud,QingkaiLu/RAMCloud,behnamm/cs244b_project,alexandermerritt/ramcloud,jcarreira/ramcloud,DavidLi2010/ramcloud,IMCG/RamCloud,Frank-Wu/RamCloud