ncmpi_wait_all called in a loop

https://github.com/E3SM-Project/scorpio/blob/b5393eceb685f4f99da234075ab582dab41d17ca/src/clib/pio_darray_int.cpp#L2180-L2185

The above code fragment calls `ncmpi_wait_all` in a loop of `nreq_blocks` times,
which may perform less efficiently than just making a single call, for example,
```
ierr = ncmpi_wait_all(file->fh, NC_REQ_ALL, NULL, NULL);
```
This flushes all the pending nonblocking requests.
FYI. `NC_REQ_ALL` was first introduced in PnetCDF 1.7.0.