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/util/mining.cpp

@@ -77,7 +77,7 @@ CTxIn MineBlock(const NodeContext& node, const CScript& coinbase_scriptPubKey)
 std::shared_ptr<CBlock> PrepareBlock(const NodeContext& node, const CScript& coinbase_scriptPubKey)
 {
     auto block = std::make_shared<CBlock>(
-        BlockAssembler{Assert(node.chainman)->ActiveChainstate(), *Assert(node.mempool)}
+        BlockAssembler{Assert(node.chainman)->ActiveChainstate(), Assert(node.mempool.get())}
             .CreateNewBlock(coinbase_scriptPubKey)
             ->block);