Alter the size of the file.
void resize( uint64_t n, asio::error_code & ec);
This function resizes the file to the specified size, in bytes. If the
current file size exceeds n
then any extra data is discarded. If the current size is less than n
then the file is extended and filled
with zeroes.
The new size for the file.
Set to indicate what error occurred, if any.