cas: increase reader iobuf size in WriteACI to 32KiB
cas: increase reader iobuf size in WriteACI to 32KiB

Consuming tar files 512 bytes at a time is silly:
 open("stage1.aci", O_RDONLY|O_CLOEXEC) = 3
 read(3, "\37\213\10\0\0\tn\210\0\377\344\375\r|\24\325\0250\16\317\354\7Y00Q\tFE\210\272("..., 512) = 512
 stat("/var/lib/rkt/tmp", ...) = 0
 getpid()                                = 14921
 open("/var/lib/rkt/tmp/925091843", O_RDWR|O_CREAT|O_EXCL|O_CLOEXEC, 0600) = 4
 read(3, "\24\231\215\257\272\361]{\316\367\206s\276\377}\316\367k\347|\277t\316\367\37\316\371~\346\234\357'"..., 512) = 512
 read(3, "\322\340\5T\25\312@\25\367\312\254\205\255\361\234\360\276\200\335\227\210\305\355P\356\36\30J]\374\217\214"..., 512) = 512
 read(3, "\254\323\235W/~\353.j\225\3745q+|c\245\3706\306/2i\nX(\337N\2644\21X"..., 512) = 512
 read(3, "#|\262\323\301x=;\331\310\304-\352\30\221\333\177\201\r\fj\203Y\210D\1\250#\10Sw\260"..., 512) = 512
 ...

krnowak/rkt,crawford/rkt,tixxdz/rkt,Yhgenomics/rkt,clearlinux/rkt,Yhgenomics/rkt,shobhit85/rkt,liugenping/rkt,dgonyeo/rkt,klizhentas/rkt,ybubnov/rkt,tv42/rkt,nhlfr/rkt,dgonyeo/rkt,krnowak/rkt,kinvolk/rkt,sstabellini/rkt,smarvin/rkt,kbrwn/rkt,kinvolk/rkt,endocode/rkt,shobhit85/rkt,robszumski/rocket,lee212/rkt,marineam/rkt,maquanyi/rkt,robszumski/rocket,il9ue/rkt,akshaykarle/rkt,intfrr/rkt,intfrr/rkt,MabinGo/rkt,xnox/rkt,s-urbaniak/rkt,chrisseto/rkt,lsm5/rkt,mike1729/rkt,adfernandes/rkt,adfernandes/rkt,chrisseto/rkt,MabinGo/rkt,linki/rkt,Shashwatsh/rkt,ChengTiesheng/rkt,michelangelo13/rkt,adfernandes/rkt,squaremo/rkt,jasonchaffee/rkt,bsmr-go/rkt,monstermunchkin/rkt,mjg59/rkt,jakon89/rkt,jzelinskie/rkt,rhvgoyal/rkt,coreos/rkt,endocode/rkt,dalanlan/rkt,Shashwatsh/rkt,luebken/rkt,klizhentas/rkt,squaremo/rkt,dalanlan/rkt,joshix/rkt,jzelinskie/rkt,ramitsurana/rkt,clearlinux/rkt,nhlfr/rkt,euank/rkt,jzelinskie/rkt,shtouff/rkt,il9ue/rkt,ramitsurana05/rkt,nhlfr/rkt,tmrts/rkt,rkt/rkt,lee212/rkt,coreos/rkt,shtouff/rkt,Shashwatsh/rkt,shtouff/rkt,joshix/rkt,sstabellini/rkt,crawford/rkt,MattDees/rkt,alban/rkt,tmrts/rkt,lucasallan/rkt,jonboulle/rkt,jakon89/rkt,markllama/rkt,rkt/rkt,sstabellini/rkt,iaguis/rkt,jakon89/rkt,xnox/rkt,akshaykarle/rkt,crawford/rkt,jjlakis/rkt,tv42/rkt,marineam/rkt,MattDees/rkt,rkt/rkt,luebken/rkt,monstermunchkin/rkt,tv42/rkt,s-urbaniak/rkt,dalanlan/rkt,mattthias/pkg-rkt,bsmr-go/rkt,tixxdz/rkt,wparad/rkt,alban/rkt,squeed/rkt,mattthias/pkg-rkt,maquanyi/rkt,tmrts/rkt,ramitsurana/rkt,mjg59/rkt,mike1729/rkt,smarvin/rkt,alexhersh/rkt,s-urbaniak/rkt,maquanyi/rkt,monstermunchkin/rkt,matthaias/rkt,Yhgenomics/rkt,alexhersh/rkt,intelsdi-x/rkt,rhvgoyal/rkt,achanda/rkt,kinvolk/rkt,wparad/rkt,achanda/rkt,yifan-gu/rocket,iankronquist/rkt,luebken/rkt,linki/rkt,kinvolk/rkt,liugenping/rkt,euank/rkt,rot26/rkt,rot26/rkt,sinfomicien/rkt,sinfomicien/rkt,cab105/rkt,jjlakis/rkt,lucasallan/rkt,jjlakis/rkt,intelsdi-x/rkt,iaguis/rkt,mattthias/pkg-rkt,jasonchaffee/rkt,lee212/rkt,squeed/rkt,Wilybald/rkt,matthaias/rkt,akshaykarle/rkt,rot26/rkt,mjg59/rkt,fdserr/rkt,sinfomicien/rkt,kbrwn/rkt,cab105/rkt,michelangelo13/rkt,sgotti/rkt,sgotti/rkt,chrisseto/rkt,markllama/rkt,yifan-gu/rkt,rhvgoyal/rkt,ChengTiesheng/rkt,Wilybald/rkt,xnox/rkt,Wilybald/rkt,krnowak/rkt,iankronquist/rkt,alexhersh/rkt,intelsdi-x/rkt,intfrr/rkt,ramitsurana05/rkt,alban/rkt,klizhentas/rkt,il9ue/rkt,clearlinux/rkt,squeed/rkt,michelangelo13/rkt,shobhit85/rkt,wparad/rkt,markllama/rkt,Wilybald/rkt,matthaias/rkt,robszumski/rocket,yifan-gu/rocket,marineam/rkt,liubq919/rkt,ybubnov/rkt,tixxdz/rkt,linki/rkt,liubq919/rkt,kbrwn/rkt,fdserr/rkt,lucasallan/rkt,jonboulle/rkt,fdserr/rkt,lsm5/rkt,MattDees/rkt,coreos/rkt,ramitsurana05/rkt,blixtra/rkt,jonboulle/rkt,squaremo/rkt,yifan-gu/rocket,iaguis/rkt,MabinGo/rkt,lsm5/rkt,dgonyeo/rkt,rkt/rkt,yifan-gu/rkt,blixtra/rkt,joshix/rkt,mike1729/rkt,liubq919/rkt,ramitsurana/rkt,ChengTiesheng/rkt,achanda/rkt,smarvin/rkt,yifan-gu/rkt,endocode/rkt,liugenping/rkt,cab105/rkt,blixtra/rkt,euank/rkt,lucasallan/rkt,iankronquist/rkt,jasonchaffee/rkt,ybubnov/rkt,bsmr-go/rkt,sgotti/rkt,joshix/rkt