Add option verify_checksums_in_compaction

Summary:
If verify_checksums_in_compaction is true, compaction will verify checksums. This is default.
If it's false, compaction doesn't verify checksums. This is useful for in-memory workloads.

Test Plan: corruption_test

Reviewers: dhruba

Reviewed By: dhruba

CC: leveldb

Differential Revision: https://reviews.facebook.net/D16695

include/rocksdb/options.h

@@ -612,6 +612,11 @@ struct Options {
   // The options needed to support Universal Style compactions
   CompactionOptionsUniversal compaction_options_universal;
 
+  // If true, compaction will verify checksum on every read that happens
+  // as part of compaction
+  // Default: true
+  bool verify_checksums_in_compaction;
+
   // Use KeyMayExist API to filter deletes when this is true.
   // If KeyMayExist returns false, i.e. the key definitely does not exist, then
   // the delete is a noop. KeyMayExist only incurs in-memory look up.