message: mining, refactor: add m_mempool.cs thread safety lock assertions
  in src/node/miner to:
  BlockAssembler::addPackageTxs()
  BlockAssembler::SkipMapTxEntry()
  BlockAssembler::UpdatePackagesForAdded()
  These functions have thread safety lock annotations in
  their declarations but are missing the corresponding
  run-time lock assertions in their definitions.
  Per doc/developer-notes.md: "Combine annotations in function
  declarations with run-time asserts in function definitions."
diff: @@ -303,6 +303,8 @@ void BlockAssembler::AddToBlock(CTxMemPool::txiter iter)
  int BlockAssembler::UpdatePackagesForAdded(const CTxMemPool::setEntries& alreadyAdded,
  indexed_modified_transaction_set &mapModifiedTx)
  {
  + AssertLockHeld(m_mempool.cs);
  +
  int nDescendantsUpdated = 0;
  for (CTxMemPool::txiter it : alreadyAdded) {
  CTxMemPool::setEntries descendants;
  @@ -339,6 +341,8 @@ int BlockAssembler::UpdatePackagesForAdded(const CTxMemPool::setEntries& already
  // cached size/sigops/fee values that are not actually correct.
  bool BlockAssembler::SkipMapTxEntry(CTxMemPool::txiter it, indexed_modified_transaction_set& mapModifiedTx, CTxMemPool::setEntries& failedTx)
  {
  + AssertLockHeld(m_mempool.cs);
  +
  assert(it != m_mempool.mapTx.end());
  return mapModifiedTx.count(it) || inBlock.count(it) || failedTx.count(it);
  }
  @@ -366,6 +370,8 @@ void BlockAssembler::SortForBlock(const CTxMemPool::setEntries& package, std::ve
  // transaction package to work on next.
  void BlockAssembler::addPackageTxs(int& nPackagesSelected, int& nDescendantsUpdated, uint32_t nTime)
  {
  + AssertLockHeld(m_mempool.cs);
  +
  // mapModifiedTx will store sorted packages after they are modified
  // because some of their txs are already in the block
  indexed_modified_transaction_set mapModifiedTx;
  