Erase all PK-based filtering structures

[sl-at-ibm]

Bringing further the motion with making table filtering uniform (= no matter what the role of the column in the table schema), this PR evolves #143 -- supplanting it if accepted -- and entirely removes the following from filter builders:

• PrimaryKeyFilter
• CompoundKey
• CompositeKey

This is only for filtering. The rationale is that:

1. users can treat all columns the same way when it comes to constructing filters, e.g. Builders<RowClass>.TableFilter.Eq(...) and this is not a problem, (functionally) resulting in the same payload to the API
2. more aligned with the other clients, and the doc structure / cross-language learning journey for the user also benefits
3. Structures that enforce the PK/schema structure of a table would be more useful, except (a) the specific Cassandra constraints on PK-based filtering are far from trivial and not easily captured in a compile-time-enforced way; and (b) which columns you can and which you can't filter on, ultimately also depends on the table indexes (and hence even more out of control from this perspective).

Note: this PR replaces #143 if merged (so, let's make sure to close that one if this is accepted).

Closes #75