sstables: close input_streams opened in random_access_reader
sstables: close input_streams opened in random_access_reader

Read-ahead requires input_streams to be closed, so add a close() method
to random_access_reader() and use it.

We don't yet call actually input_stream::close(), because that is not yet
merged; this patch lays the groundwork.

senseb/scylla,victorbriz/scylla,dwdm/scylla,gwicke/scylla,bowlofstew/scylla,dwdm/scylla,capturePointer/scylla,acbellini/scylla,capturePointer/scylla,aruanruan/scylla,asias/scylla,gwicke/scylla,duarten/scylla,rentongzhang/scylla,aruanruan/scylla,phonkee/scylla,wildinto/scylla,tempbottle/scylla,tempbottle/scylla,wildinto/scylla,rluta/scylla,duarten/scylla,rentongzhang/scylla,aruanruan/scylla,capturePointer/scylla,justintung/scylla,eklitzke/scylla,scylladb/scylla,senseb/scylla,phonkee/scylla,linearregression/scylla,victorbriz/scylla,scylladb/scylla,linearregression/scylla,avikivity/scylla,scylladb/scylla,guiquanz/scylla,linearregression/scylla,duarten/scylla,senseb/scylla,glommer/scylla,stamhe/scylla,respu/scylla,justintung/scylla,guiquanz/scylla,eklitzke/scylla,glommer/scylla,respu/scylla,kjniemi/scylla,tempbottle/scylla,respu/scylla,gwicke/scylla,kangkot/scylla,raphaelsc/scylla,avikivity/scylla,raphaelsc/scylla,kangkot/scylla,shaunstanislaus/scylla,justintung/scylla,stamhe/scylla,stamhe/scylla,avikivity/scylla,kjniemi/scylla,asias/scylla,shaunstanislaus/scylla,wildinto/scylla,eklitzke/scylla,guiquanz/scylla,bowlofstew/scylla,kjniemi/scylla,acbellini/scylla,rentongzhang/scylla,kangkot/scylla,dwdm/scylla,asias/scylla,rluta/scylla,victorbriz/scylla,acbellini/scylla,raphaelsc/scylla,rluta/scylla,shaunstanislaus/scylla,phonkee/scylla,bowlofstew/scylla,glommer/scylla,scylladb/scylla