[TAP] Fix the TAP_SCHEMA mapping.

When defining in the configuration file a different name for TAP_SCHEMA content,
the service implementor was also forced to define the same mapping in the
database with the column `dbName`.

This is no longer necessary. From now on, the `dbName` column will be ignored
for all standard TAP_SCHEMA content. Instead, the name specified in the
configuration file (if any) will be used instead. This way, the mapping for
standard TAP_SCHEMA content is only specified once and at only one place:
the configuration file.

_This commit resolves the GitHub issue #98_