output_stream: coalesce large buffers with data already in the buffer
output_stream: coalesce large buffers with data already in the buffer

Assuming the output_stream size is set to 8K, a sequence of writes of
lengths: 128B, 8K, 128B would yield three fragments of exactly those
sizes. This is not optimal as one could fit those in just 2 fragments
of up to 8K size. This change makes the output_stream yield 8K and
256B fragments for this case.

bowlofstew/seastar,tempbottle/seastar,anzihenry/seastar,jonathanleang/seastar,kangkot/scylla,bowlofstew/seastar,sjperkins/seastar,avikivity/seastar,chunshengster/seastar,duarten/scylla,bzero/seastar,bzero/seastar,xtao/seastar,joerg84/seastar,respu/scylla,raphaelsc/scylla,stamhe/scylla,wildinto/scylla,acbellini/seastar,wildinto/seastar,chunshengster/seastar,raphaelsc/seastar,justintung/scylla,avikivity/scylla,xtao/seastar,asias/scylla,raphaelsc/scylla,bowlofstew/seastar,kjniemi/scylla,capturePointer/scylla,phonkee/scylla,wildinto/scylla,duarten/scylla,kjniemi/scylla,tempbottle/scylla,justintung/scylla,kjniemi/scylla,cloudius-systems/seastar,leejir/seastar,joerg84/seastar,slivne/seastar,slivne/seastar,dwdm/seastar,justintung/scylla,dwdm/seastar,respu/scylla,scylladb/scylla,stamhe/seastar,printedheart/seastar,eklitzke/scylla,scylladb/scylla,mixja/seastar,flashbuckets/seastar,slivne/seastar,scylladb/scylla-seastar,kjniemi/seastar,leejir/seastar,norcimo5/seastar,avikivity/seastar,printedheart/seastar,hongliangzhao/seastar,tempbottle/seastar,joerg84/seastar,stamhe/scylla,koolhazz/seastar,raphaelsc/seastar,sjperkins/seastar,gwicke/scylla,acbellini/seastar,acbellini/seastar,dreamsxin/seastar,wildinto/seastar,mixja/seastar,bowlofstew/scylla,phonkee/scylla,shaunstanislaus/scylla,linearregression/seastar,sjperkins/seastar,gwicke/scylla,rentongzhang/scylla,syuu1228/seastar,scylladb/seastar,flashbuckets/seastar,flashbuckets/seastar,dwdm/scylla,hongliangzhao/seastar,tempbottle/seastar,bowlofstew/scylla,kjniemi/seastar,tempbottle/scylla,norcimo5/seastar,gwicke/scylla,xtao/seastar,jonathanleang/seastar,acbellini/scylla,aruanruan/scylla,tempbottle/scylla,shaunstanislaus/scylla,glommer/scylla,cloudius-systems/seastar,guiquanz/scylla,shyamalschandra/seastar,asias/scylla,koolhazz/seastar,dwdm/scylla,syuu1228/seastar,rentongzhang/scylla,linearregression/scylla,linearregression/scylla,scylladb/seastar,wildinto/seastar,scylladb/scylla-seastar,scylladb/scylla,avikivity/scylla,koolhazz/seastar,jonathanleang/seastar,eklitzke/scylla,wildinto/scylla,rentongzhang/scylla,victorbriz/scylla,kangkot/scylla,dreamsxin/seastar,asias/scylla,eklitzke/scylla,kjniemi/seastar,cloudius-systems/seastar,rluta/scylla,guiquanz/scylla,rluta/scylla,kangkot/scylla,linearregression/seastar,stamhe/seastar,dwdm/scylla,shaunstanislaus/scylla,victorbriz/scylla,anzihenry/seastar,linearregression/seastar,anzihenry/seastar,glommer/scylla,syuu1228/seastar,printedheart/seastar,senseb/scylla,phonkee/scylla,acbellini/scylla,capturePointer/scylla,duarten/scylla,ducthangho/imdb,ducthangho/imdb,shyamalschandra/seastar,dreamsxin/seastar,guiquanz/scylla,leejir/seastar,rluta/scylla,bowlofstew/scylla,bzero/seastar,shyamalschandra/seastar,scylladb/seastar,ducthangho/imdb,chunshengster/seastar,mixja/seastar,senseb/scylla,senseb/scylla,capturePointer/scylla,scylladb/scylla-seastar,norcimo5/seastar,avikivity/scylla,aruanruan/scylla,stamhe/seastar,scylladb/scylla,glommer/scylla,stamhe/scylla,respu/scylla,acbellini/scylla,hongliangzhao/seastar,victorbriz/scylla,raphaelsc/seastar,linearregression/scylla,aruanruan/scylla,avikivity/seastar,dwdm/seastar,raphaelsc/scylla