MergingIterator.Seek() to lazily initialize MinHeap

Summary:
For the use cases that prefix filtering is enabled, initializing heaps when doing MergingIterator.Seek() might introduce non-negligible costs. This patch makes it lazily done.

Test Plan: make all check

Reviewers: haobo,dhruba,kailiu

CC:

Task ID: #

Blame Rev:
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
A Simple Plain Table

Summary:
A Simple plain table format. No block structure. When creating the table reader, scanning the full table to create indexes.

Test Plan:Add unit test

Reviewers:haobo,dhruba,kailiu

CC:

Task ID: #

Blame Rev:
Fix a memory leak happened in table_test
[Only for Performance Branch] A Hacky patch to lazily generate memtable key for prefix-hashed memtables.

Summary:
For prefix mem tables, encoding mem table key may be unnecessary if the prefix doesn't have any key. This patch is a little bit hacky but I want to try out the performance gain of removing this lazy initialization.

In longer term, we might want to revisit the way we abstract mem tables implementations.

Test Plan: make all check

Reviewers: haobo, igor, kailiu

Reviewed By: igor

CC: leveldb

Differential Revision: https://reviews.facebook.net/D14265