chert alldocspostlist should use posting table instead of termlist table

[Richard Boulton]

The alldocspostlist for chert currently uses the termlist table to get the list of documents. However, chert now stores the document length for every document in a single list in the termlist table, so this should be used instead. Not only would this help in the target of making the termlist table optional, I believe it would also involve less IO, and should make pure ValueRange searches faster, since these make use of an alldocspostlist to identify document IDs which exist.

[Richard Boulton]

Implemented in revision [10716].