Ideally our methods and functions should do one thing, do that well,
and do only that.  add_table_to_list does far more than adding a
table to a list, so this commit factors the TABLE_LIST creation out
to a new TABLE_LIST constructor.  It then uses placement new()
to create it in the correct memory area (result of thd->calloc).
Benefits of this approach:
 1. add_table_to_list now returns as early as possible on an error
 2. fewer side-effects incurred on creating the TABLE_LIST object
 3. TABLE_LIST won't be calloc'd if copy_to_db fails
 4. local declarations moved closer to their respective first uses
 5. improved code readability and logical flow
Also factored a couple of other functions to keep the happy path
more to the left, which makes them easier to follow at a glance.