Differentiate lazy parsing of header and transactions in blocks

[stevenroose]

Currently, BitcoinSerialization has a _needInstance() method to indicate that parsing is required.

In blocks, however, parsing all transactions (even if done lazily), requires significantly more work than parsing only the header, while for many properties of the block, only the header is required.

Proposed solution:
Introduce a _needHeader() that only deserializes the header and indicates that the header has been deserialized so that following calls to this method will return.
Follow the same methods to ensure the serialization stays consistent => see BitcoinSerialization._needInstance().

The same for _needTransactions().