Send the same getblocktemplate RPC response until the template would change

[teor2345]

Motivation

str4d told us that zcashd caches its getblocktemplate RPC response when the RPC is called frequently. This can improve RPC performance, which is important for miner profits.

The getblocktemplate response must change when:

• the best tip hash changes (Zebra's ChainTip watch channel)
• the max time is reached

The getblocktemplate response can also change, but is not required to change, when:

• the mempool adds or evicts transactions

Designs

• Cache the getblocktemplate RPC response based on the long polling ID
• Delete the cached RPC value if the long polling ID would change
• If there is a cached value, return it when the RPC is called
• Don't cache invalid templates (like #6037)
  • Mark invalid templates in the cache so we don't try to re-send or re-check them
  • Return a RPC error if we don't find a valid template within a target block interval or two

Related Work

This will be easy to implement after long polling support in #5720