Handmade blockstore

[Bulat-Ziganshin]

I just come up with efficient and reliable (?) handmade blockstore architecture (note that all blocks in a single store have the same size):

1. Blocks are written sequentially into one or few large files
2. We keep in-memory map for cid->offset and list of free blocks (holes after deletion)
3. A separate file keeps log of blocks written, i.e. the same cid->offset pairs and offsets freed
4. At the program restart the log is read to restore in-memory map+list and new log is created that drops overwritten records
5. The log need to be flushed only when we are going to sign a contract, otherwise blocks not included in the log are considered free