Add keys to "temp" tables.

[scottcain]

The tmp_gff_load_cache table is a not-really-temporary table that the GFF3 bulk loader uses to keep track of feature names and IDs. To make database replication easier, it should have a primary key or unique index.

The gff_meta table is also without either a primary key or a unique index. So, for replication of the DB (even though this table may be "disposable"), it too needs either a primary key or a unique index.

This is migrated from the bug tracker at Sourceforge: http://sourceforge.net/p/gmod/bugs/363/