Allow Reading of Irreversible Block Headers

[mvandeberg]

The block log currently only returns entire blocks. There are scenarios in which the block header contains all the information required and is much faster to read.

The block log should be expanded to allow reading of just a header. This is trivial as the FC serialization appends children classes at the end of the parent. Deserializing the parent class is as simple as unpacking the parent from the same data source.