Fix concurrent schema-changes

[markhannum]

    Introduce sc_add_drop_race test which executes multiple concurrent schema-
    changes against random, sometimes overlapping tables and fix the issues
    exposed by this test:

    * sc_tables_hash prevents concurrent schema-changes on different tables
    * form_file_name can return DEADLOCK, which calling functions should handle
    * ignore already_finalized flag on abort-add-table
    * includes additional instrumentation that was required to diagnose

Signed-off-by: Mark Hannum <[email protected]>