disable the log_number check in Recover()

Summary:
There is a chance that an old MANIFEST is corrupted in 2.7 but just not noticed.
This check would fail them. Change it to log instead of returning a
Corruption status.

Test Plan: make

Reviewers: haobo, igor

Reviewed By: igor

CC: leveldb

Differential Revision: https://reviews.facebook.net/D16923

db/db_impl.cc

@@ -449,6 +449,7 @@ uint64_t DBImpl::TEST_Current_Manifest_FileNo() {
 
 Status DBImpl::NewDB() {
   VersionEdit new_db;
+  new_db.SetVersionNumber();
   new_db.SetComparatorName(user_comparator()->Name());
   new_db.SetLogNumber(0);
   new_db.SetNextFile(2);