Short circuit predicates on dictionary-coded columns

In the common case that a column has no updates in a given DRS, if we see that no entries in the dictionary match the predicate, we can short circuit at a few layers:

• we can store a flag in the cfile footer that indicates that all blocks are dict-coded (ie there are no fallbacks). In that case, we can skip the whole rowset
• if a cfile is partially dict-encoded, we can skip any dict-coded blocks without decoding the dictionary words