Working with headers

We need more functionality for block storage

- [ ] Working with headers. We need to be able to read header without pulling complete block (block on #574, since it introduce way to turn block to header)
- [ ] Reading of block index on start. We need to read headers of all blocks on start. Also in order to build index we need to sort them in topological order. Should we add height field to table to be able to sort them this way?
- [ ] Decide whether we store only complete blocks or should we store only headers. Light clients will only store headers but light clients are only in the future.
- [ ] Handling of bad blocks. Could we end up storing invalid blocks? How should we mark invalid blocks?