Handle large docs via GridFS

[vkuznet]

It is possible that DAS will received a doc whose size will exceed MongoDB limit (4MB by default). In that case the bulk insert will fail for all docs in insert sequence (due to generators). To avoid that I need a new generator routine whose purpose will be scan doc and pass it if it has size < 4MB or put it to GridFS.

[vkuznet]

valya: (In 01601f8) Fix DBS2 bug; add support for GridFS; add new generator parse2gridfs who pass input docs or write them to GridFS and replace their meta-data with gridfs pointer. fixes #611

Signed-off-by: Valentin Kuznetsov vkuznet@gmail.com