[leo_gateway][nfs] Uploading a big file may take too long

[mocchira]

It turned out that
The more large size, the more head requests a leo_gateway issues to leo_storage(s).
The below code may cause this problem in case handling a very large file.
https://github.com/leo-project/leo_gateway/blob/1.3.0/src/leo_nfs_file_handler.erl#L456-L479

This can be solved decreasing the number of requests to leo_storage(s).
In order to realise the above requirement, we need to come up with another implementation to calculate the total file size after updating.

[mocchira]

As the current implementation assumes a chunk size set to files is same with the one in leo_gateway.conf (see: #540 )
the total file size can be calculated using simple math.

chunk_size * (cnumber - 1) + dsize_of_last_chunk.