Updates to db metadata

[havok2063]

This PR is related to sdss/valis#57 and updates the client-side store functions to better handle non-unique database metadata. With PR sdss/valis#57 the response from info/database now returns a dictionary of schema, with keys {table}.{column} whereas before it was organized by {column} only. The front-end now organizes the database metadata by {schema}.{table}.{column} for more unique keys, for cases where multiple table and schema have the same column name, but different metadata. The store function store.get_field_from_db now accepts an optional context object specifying a preferred schema or table to use when looking up the field metadata. e.g. store.get_field_from_db('sdss_id', 'description', {'schema': 'vizdb'}) will pull the description of the sdss_id column from the best matching column in the vizdb schema.