fix(gridfs): make a copy of chunk before writing to server
fix(gridfs): make a copy of chunk before writing to server

A shared buffer is used for gridfs uploads, but there is a race
where the buffer could be changed before the write has a chance to
execute. Instead, we cut a copy of the buffer for use during the
write operation.

mongodb/node-mongodb-native,mongodb/node-mongodb-native,mongodb/node-mongodb-native,mongodb/node-mongodb-native