[WIP] Use B+ tree for manifest structure.

[Little-Wallace]

Problem

part of #110
Now every read thread will acquire read lock for LevelControl for creating sst iterator. And if we update the files in one level, we need sort all files while holding a write lock and it will block all read thread. I hope I can propose a new structure so that we can update the sst file quickly so that it will not block other read thread too long.

Design

I use a four-level B+ Tree. If a compact job want to add or remove some files in one level (mostly the update files are continuously), it will copy every page it updates (for a four-level B+ Tree, it will copy and replace three pages). There are only tens record of every page, so that the update operation can be very fast.

[skyzh]

Looks like a good idea! As I'm busy with exams this month, I'll take a look at this at the end of October.