Relax column tag uniqueness requirement

[fulghum]

Instead of requiring column tags to be unique across all tables, this change only requires column tags to be unique within each table.

Column tags are intended to be an internal only, implementation detail of Dolt, but if a conflict occurred (two columns in the same branch having the same column tag), then it blocked customers and often required expert help to resolve. This change greatly reduces the chance of hitting a column tag conflict, since it reduces the scope across which a column tag must be unique.

[coffeegoddd]

@fulghum DOLT

comparing_percentages
100.000000 to 100.000000

version	result	total
9114669	ok	5937471

version	total_tests
9114669	5937471

correctness_percentage
100.0