Add updated_at and create_at to mesh data tables
Add primary key to mesh_term table

The timestamps are not much use if there is no primary key preventing
collisions.
Add primary key to mesh term table install as in the migration in [97b4d14117391b1e68d7cbd45b8c46f8fd285d69]
Merge pull request #642 from jrjohnson/302-meshtimestamp

Add updated_at and create_at to mesh data tables