Add more black-box tests for PlainTable and explicitly support total order mode

Summary:
1. Add some more implementation-aware tests for PlainTable
2. move from a hard-coded one index per 16 rows in one prefix to a configurable number. Also, make hash table ratio = 0  means binary search only. Also fixes some divide 0 risks.
3. Explicitly support total order (only use binary search)
4. some code cleaning up.

Test Plan: make all check

Reviewers: haobo, kailiu

Reviewed By: haobo

CC: leveldb

Differential Revision: https://reviews.facebook.net/D16023

Author: siying

db/db_impl.cc

@@ -2754,7 +2754,10 @@ Iterator* DBImpl::NewInternalIterator(const ReadOptions& options,
 
 Iterator* DBImpl::TEST_NewInternalIterator() {
   SequenceNumber ignored;
-  return NewInternalIterator(ReadOptions(), &ignored);
+  ReadOptions read_options;
+  // Use prefix_seek to make the test function more useful.
+  read_options.prefix_seek = true;
+  return NewInternalIterator(read_options, &ignored);
 }
 
 std::pair<Iterator*, Iterator*> DBImpl::GetTailingIteratorPair(