Prevent infinite loop in db fail handler
Fix issue with capitalized table prefixes in plugins

The init_queries plugin function now receives table names in original case.
Potentially breaking change if qa_db_add_table_prefix is manually wrapped with strtolower, however, I have not found any code in the wild that would break from this change.
Q2A core code also now using case-sensitive comparisons (there were several instances of tables not being normalized anyway).
Fixes #45.