Specifically:
- reduce the number of realloc's and malloc's by moving
some arrays out of the cycle loop, if we know that there
size is not changing
- store the rank of the aggregator in a separate variable to avoid
continuous dereferencing
- change the wait_all logic in write_all to use a fix number of requests
(even if they are MPI_REQUEST_NULL)
- fix the timing to considere the two initial allgather and the one
allgatherv operation to be a part of it
- add more comments.
c2c44b11dc