Remove randomness from AstroDB.create_unique_table_name()

[AustinCullar]

I don't like that the AstroDB.create_unique_table_name() method generates random strings and hopes that they are unique, regardless of how unlikely collisions might be (1 in 26^12 for a 12 character string of capital letters).

I believe I can modify this method to work a bit like an odometer. I'd like to use 10 character strings of capital letters, but roll the string forward with each additional comment table name generated. For example:

Comment table 1: AAAAAAAAAA
Comment table 2: BAAAAAAAAA
Comment table 3: CAAAAAAAAA
...

This would provide a large set of possible table names (26^10) while removing the uncertainty involved with random string generation.

This will also require updating the tests in tests/test_astro_db.py.