Chain DB: Support tentative header

At the moment the only state changing API call in the chain DB is `addBlock`, which then triggers chain selection, either adding new blocks to the existing chain or changing to a fork. This in turn then notifies all the `Reader`s. This introduces propagation delays for headers through the network:

* Node A sends header 1 to Node B
* Node B requests the corresponding block 1 from node A
* After node B downloads the block and adds it to its own chain, it updates its readers
* It can now send header 1 to node C

It would be better if we could avoid block-download delay before sending the header to node C. We can do this by adding a (single) "tentative header" to the chain database. This would be the header of the block that fits (directly) on top of the current chain. This state would be _set_ by the chain sync client, but managed by the chain DB because it affects the readers, and needs to be updated whenever `addBlock`/chain selection takes place.