sstables: Dynamically adjust I/O size for index reads

Currently, index reader uses 128 KiB I/O size with read-ahead. That is
a waste of bandwidth if index entries contain large promoted index and
binary search will be used within the promoted index, which may not
need to access as much.

The read-ahead is wasted both when using binary search and when using
the scanning cursor.

On the other hand, large I/O is optimal if there is no promoted index
and we're going to parse the whole page.

There is no way to predict which case it is up front before reading
the index.

Attaching dynamic adjustments (per-sstable) lets the system auto adjust
to the workload from past history.

The large promoted index workload will settle on reading 32 KiB (with
read-ahead). This is still not optimal, we should lower the buffer
size even more. But that requires a seastar change, so is deferred.

sstables/index_reader.hh

@@ -360,6 +360,7 @@ class index_reader {
             options.buffer_size = sst->sstable_buffer_size;
             options.read_ahead = 2;
             options.io_priority_class = pc;
+            options.dynamic_adjustments = sst->_index_history;
             return options;
         }
 

sstables/sstables.hh

@@ -488,6 +488,7 @@ private:
 
     lw_shared_ptr<file_input_stream_history> _single_partition_history = make_lw_shared<file_input_stream_history>();
     lw_shared_ptr<file_input_stream_history> _partition_range_history = make_lw_shared<file_input_stream_history>();
+    lw_shared_ptr<file_input_stream_history> _index_history = make_lw_shared<file_input_stream_history>();
 
     //FIXME: Set by sstable_writer to influence sstable writing behavior.
     //       Remove when doing #3012