mysql: optimize the tagMap mechanism

[llorllale]

The tagMap mechanism of the mysql storage implementation should be optimized. It is being stored as a composite value of a single record in the data table. Instead, the tags should be maintained in a separate 'tags' table. Queries, updates, and deletions involving the tagMap should not require SELECT prior to UPDATE/DELETE. Furthermore, it should be possible to do bulk updates/deletions to the tag map (see Batch()).

[DRK3]

@llorllale Note this issue here: #88

Really the entire "tag map" mechanism should be replaced with columns + SQL indices. The "tag map" was just a "quick and dirty" solution to allow it to function per the interface requirements.

[llorllale]

OK - closed as this is a duplicate of #88