`DataTypeFactory::get` locks a mutex inside shared a context, there was a case when many threads were locked inside it

https://pastila.nl/?0043b3e6/c557368f4cf0eb87a3c6f2cc162bc693

It is not clear which thread locked the mutex for a long time. However, it may be possible to lock less inside `DataTypeFactory::get`.
{
    if (isGlobalContext())
        throw Exception(ErrorCodes::LOGICAL_ERROR, "Global context cannot have external tables");
    auto lock = getLock();
    if (external_tables_mapping.end() != external_tables_mapping.find(table_name))
        throw Exception("Temporary table " + backQuoteIfNeed(table_name) + " already exists.", ErrorCodes::TABLE_ALREADY_EXISTS);
    external_tables_mapping.emplace(table_name, std::make_shared<TemporaryTableHolder>(std::move(temporary_table)));
}
std::shared_ptr<TemporaryTableHolder> Context::removeExternalTable(const String & table_name)