Correctly handle "unique" params in filters

[sileix]

Currently, once a unique param is used in filter, the table is marked as no_filter, i.e., no more filter can be added to it. However, in some cases, this is not reasonable:

1. or operator: this should always be allowed even if unique param is used
2. not operator on unique param: this should turn the filter to be not unique (?)

[gcampax]

Also, if the filter value is not a constant

[sileix]

Also, if the operand is not "equal"