explicit naming of memtables

This requires us to support overwriting, if someone decides to name two different memtables the same. The implementation of this is pretty dicey.

solution: memtables are always unique, and if you want to name them you have to use create_table.

weakref.finalize cleanup of underlying data

This depends on safe execution of the Python GC at any time, and it's possible to deadlock the interpreter with psycopg using a finalizer that executes a DROP TABLE query.

solution: atexit is probably the best we can do

For backends that use temporary tables we can remove finalizers altogether.