Summary:
I have implemented a FreeList version that supports fragmented blob chunks. Each block gets allocated and freed in FIFO order. Since the idea for the blocks to be big, we will not take a big hit of non-sequential IO. Free list is also faster, taking only O(k) size in both free and allocate instead of O(N) as before.

See more info on the task: https://our.intern.facebook.com/intern/tasks/?t=2990558

Also, I'm taking Slice instead of const char * and size in Put function.

Test Plan: unittests

Reviewers: haobo, kailiu, dhruba, emayanke

Reviewed By: dhruba

CC: leveldb

Differential Revision: https://reviews.facebook.net/D13569