Skip dictionary and collection conjunct assignment for non-Parquet scans.

In HdfsScanNode.init() we try to assign dictionary and collection conjuncts even for non-Parquet scans. Such predicates only make sense for Parquet scans, so there is no point in collecting them for other scans.

The current behavior is undesirable because:

• init() can be substantially slower because assigning dictionary filters may involve evaluating exprs in the BE which can be expensive
• the explain plan of non-Parquet scans may have a section "parquet dictionary predicates" which is confusing/misleading

Relevant code snippet from HdfsScanNode:

@Override
  public void init(Analyzer analyzer) throws ImpalaException {
    conjuncts_ = orderConjunctsByCost(conjuncts_);
    checkForSupportedFileFormats();

    assignCollectionConjuncts(analyzer);
    computeDictionaryFilterConjuncts(analyzer);

    // compute scan range locations with optional sampling
    Set<HdfsFileFormat> fileFormats = computeScanRangeLocations(analyzer);
...
    if (fileFormats.contains(HdfsFileFormat.PARQUET)) { <--- assignment should go in here
      computeMinMaxTupleAndConjuncts(analyzer);
    }
...
}