refactor: pass BlockCreateOptions to createNewBlock

Rather than pass options individually to createNewBlock and then
combining them into BlockAssembler::Options, this commit introduces
BlockCreateOptions and passes that instead.

Currently there's only one option (use_mempool) but the next
commit adds more.

Co-authored-by: Ryan Ofsky <ryan@ofsky.org>

src/node/miner.cpp

@@ -66,7 +66,7 @@ static BlockAssembler::Options ClampOptions(BlockAssembler::Options options)
 
 BlockAssembler::BlockAssembler(Chainstate& chainstate, const CTxMemPool* mempool, const Options& options)
     : chainparams{chainstate.m_chainman.GetParams()},
-      m_mempool{mempool},
+      m_mempool{options.use_mempool ? mempool : nullptr},
       m_chainstate{chainstate},
       m_options{ClampOptions(options)}
 {