Code cleanup for dynamic read_all and write_all

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.

Author: edgargabriel