freetrader authored May 04, 2020 and Calin Culianu committed May 04, 2020

Summary:
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Two new light-weight RPC calls were added:  `getblocktemplatelight` and `submitblocklight`.
These RPCs reduce the round-trip time for mining software when retrieving new block templates.
Transaction data is never sent between mining software and `bitcoind`.  Instead, `job_id`'s
are returned and `bitcoind` later reconstructs the full block based on this `job_id` and the
solved header + coinbase submitted by the miner, leading to more efficient mining.

This is the specification for these RPC calls.

Test plan:
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- Review