save_file should store the file etag hash so that it doesn't need to be calculated on each read

[ShaneHarvey]

In #170 we remove md5 hashing and updated send_file to dynamically calculate the sha1 hash on each call. This is expensive because it requires reading the full GridFS file and hashing the contents to set the etag, then we read the file in full again to send the file itself.

It would be better for save_file to add the etag hash to the GridFS file metadata so that it doesn't need to be recalculated on each send_file call.

[ShaneHarvey]

We should also check if send_file() can be used on preexisting GridFS files, ie files not uploaded via save_file(). In that case we may need to keep the current lazy sha1 calculation as a backup when the hash metadata field is missing.