As we moved most of the creation of a BufferPool to be via the
Streaming::pool() method, which uses a thread-local, it makes sense
to start cleaning up the design and make it more modern C++.
The above mentioned method would return a reference and you'd see
loads of places use `auto &pool =` which is less than ideal.
As the number of places where we actually instantiate a BufferPool
goes down, the usage of some sort of smart pointer makes more sense.
This now makes all APIs use BufferPool be wrapped in a shared_ptr.
Flowee experimented with a 'metablock' a piece of data stored next to
the blocks with some metadata it discovered during validation.
One of those pieces of info is the txid, which turned out to be too big
to store here (16GB or so added).
This change removes the txid but it does keep the script-flags and the
fees for each transaction (8 bytes per tx) which adds maybe 3GB to the
entire chain.
tomFlowee