Client: counting number of matching entries when data_returned is not available

[ml-evs]

Since we added a default timeout to the data_returned field (which can require a whole COLSCAN for every page of results), it is now tricky to count the number of results returned by a query unless the whole database fits in memory. This was already the case for some other databases, since data_returned is optional.

The client should specialise the --count method in this case and perform a binary search for the last accessible page matching a query. This method should require significantly fewer requests than exhaustive pagination.