Inline a couple of functions and put one save lazily clearing

Summary:
Machine several functions inline.
Also, in DBIter.Seek() make value cleaning up lazily done.
These are for the use case that Seek() are called lots of times but few return values.

Test Plan: make all check

Differential Revision: https://reviews.facebook.net/D14217

Author: siying

db/db_iter.cc

@@ -426,13 +426,13 @@ void DBIter::FindPrevUserEntry() {
 }
 
 void DBIter::Seek(const Slice& target) {
-  direction_ = kForward;
-  ClearSavedValue();
   saved_key_.clear();
   AppendInternalKey(
       &saved_key_, ParsedInternalKey(target, sequence_, kValueTypeForSeek));
   iter_->Seek(saved_key_);
   if (iter_->Valid()) {
+    direction_ = kForward;
+    ClearSavedValue();
     FindNextUserEntry(false /*not skipping */);
   } else {
     valid_ = false;