The global TAP schema collects information on
tables and columns from known TAP servers. This facilitates locating
queriable data by physics (via UCD) or keywords (via description).
Note that this shouldn't really be necessary as all information
present here should be exposed through Registry records. However,
in reality data providers currently are much more liable to give
column metadata in their tap_schema than in their Registry records.
Hence, for the time being, we maintain this service by harvesting
tap_schemas about monthly.
