miner: Make mempool optional for BlockAssembler

...also adjust callers

Changes:

- In BlockAssembler::CreateNewBlock, we now only lock m_mempool->cs and
  call addPackageTxs if m_mempool is not nullptr
- BlockAssembler::addPackageTxs now takes in a mempool reference, and is
  annotated to require that mempool's lock.
- In TestChain100Setup::CreateBlock and generateblock, don't construct
  an empty mempool, just pass in a nullptr for mempool

src/test/miner_tests.cpp

@@ -52,7 +52,7 @@ BlockAssembler MinerTestingSetup::AssemblerForTest(const CChainParams& params)
 
     options.nBlockMaxWeight = MAX_BLOCK_WEIGHT;
     options.blockMinFeeRate = blockMinFeeRate;
-    return BlockAssembler{m_node.chainman->ActiveChainstate(), *m_node.mempool, options};
+    return BlockAssembler{m_node.chainman->ActiveChainstate(), m_node.mempool.get(), options};
 }
 
 constexpr static struct {