Add block caching by hash and height.

[evoskuil]

Similar in concept to utxo cache.
Useful as performance optimization in both validation and server API.
Cannot be easily applied to address history indexes, since key-value map changes over time.
Could be applied to spend index, but slightly more complex than tx/block.
Indexes are already efficient (apart from stealth), so the benefit is much smaller.
Transactions queries are already efficient as the data is typically located in a single page.
Blocks are very expensive even compared to txs, so the benefit is greatest.

[evoskuil]

Closing as not likely to be a material optimization with the exception of the last announced block.