A big block chain is a linked list, and we pretty much need random
access to them. This should theoretically make accessing a random
point in the chain O(log2 n) instead of O(n) (with disk access scaling
based on the size of the read/write, not its location). It
theoretically takes O(n) memory based on the size, but it can do
better if the chain isn't very fragmented (which I believe will
generally be the case for long chains). It also involves fetching all
the big block locations when we open the chain, but we already do that
anyway (and it should be faster to read it all in one go than
piecemeal).