Support taking a configurable number of files from the same level to compact in a single compaction run.

Summary:
The compaction process takes some files from LevelK and
merges it into LevelK+1. The number of files it picks from
LevelK was capped such a way that the total amount of
data picked does not exceed the maxfilesize of that level.
This essentially meant that only one file from LevelK
is picked for a single compaction.

For bulkloads, we would like to take many many file from
LevelK and compact them using a single compaction run.

This patch introduces a option called the 'source_compaction_factor'
(similar to expanded_compaction_factor). It is a multiplier
that is multiplied by the maxfilesize of that level to arrive
at the limit that is used to throttle the number of source
files from LevelK.  For bulk loads, set source_compaction_factor
to a very high number so that multiple files from the same
level are picked for compaction in a single compaction.

The default value of source_compaction_factor is 1, so that
we can keep backward compatibilty with existing compaction semantics.

Test Plan: make clean check

Reviewers: emayanke, sheki

Reviewed By: emayanke

CC: leveldb

Differential Revision: https://reviews.facebook.net/D6867

include/leveldb/options.h

@@ -238,6 +238,15 @@ struct Options {
   // (expanded_compaction_factor * targetFileSizeLevel()) many bytes.
   int expanded_compaction_factor;
 
+  // Maximum number of bytes in all source files to be compacted in a
+  // single compaction run. We avoid picking too many files in the 
+  // source level so that we do not exceed the total source bytes
+  // for compaction to exceed
+  // (source_compaction_factor * targetFileSizeLevel()) many bytes.
+  // Default:1, i.e. pick maxfilesize amount of data as the source of
+  // a compaction.
+  int source_compaction_factor;
+
   // Control maximum bytes of overlaps in grandparent (i.e., level+2) before we
   // stop building a single file in a level->level+1 compaction.
   int max_grandparent_overlap_factor;