Introduce CompactBlockInfo.

This is a minimum representation of a block in memory, without all of the extra fields that BlockInfo has. A further CL will explicitly store this in the quarantine, rather than BlockHeader*. This will allow O(1) calls to get the block size and hash.

BUG=
R=sebmarchand@chromium.org

Review URL: https://codereview.appspot.com/165930043