Make some variables configurable for each db instance

Summary:
Make configurable 'targetFileSize', 'targetFileSizeMultiplier',
'maxBytesForLevelBase', 'maxBytesForLevelMultiplier',
'expandedCompactionFactor', 'maxGrandParentOverlapFactor'

Test Plan: N/A

Reviewers: dhruba

Reviewed By: dhruba

Differential Revision: https://reviews.facebook.net/D3801

db/dbformat.h

@@ -16,30 +16,6 @@
 
 namespace leveldb {
 
-// Grouping of constants.  We may want to make some of these
-// parameters set via options.
-namespace config {
-static const int kNumLevels = 7;
-
-// Level-0 compaction is started when we hit this many files.
-static const int kL0_CompactionTrigger = 4;
-
-// Soft limit on number of level-0 files.  We slow down writes at this point.
-static const int kL0_SlowdownWritesTrigger = 8;
-
-// Maximum number of level-0 files.  We stop writes at this point.
-static const int kL0_StopWritesTrigger = 12;
-
-// Maximum level to which a new compacted memtable is pushed if it
-// does not create overlap.  We try to push to level 2 to avoid the
-// relatively expensive level 0=>1 compactions and to avoid some
-// expensive manifest file operations.  We do not push all the way to
-// the largest level since that can generate a lot of wasted disk
-// space if the same key space is being repeatedly overwritten.
-static const int kMaxMemCompactLevel = 2;
-
-}  // namespace config
-
 class InternalKey;
 
 // Value types encoded as the last component of internal keys.