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/fuzz/tx_pool.cpp

@@ -97,7 +97,7 @@ void Finish(FuzzedDataProvider& fuzzed_data_provider, MockedTxPool& tx_pool, CCh
         BlockAssembler::Options options;
         options.nBlockMaxWeight = fuzzed_data_provider.ConsumeIntegralInRange(0U, MAX_BLOCK_WEIGHT);
         options.blockMinFeeRate = CFeeRate{ConsumeMoney(fuzzed_data_provider, /*max=*/COIN)};
-        auto assembler = BlockAssembler{chainstate, *static_cast<CTxMemPool*>(&tx_pool), options};
+        auto assembler = BlockAssembler{chainstate, &tx_pool, options};
         auto block_template = assembler.CreateNewBlock(CScript{} << OP_TRUE);
         Assert(block_template->block.vtx.size() >= 1);
     }