Blocking directives in mesh migration

[jbadger95]

Mesh migration can be rather slow on large distributed meshes because each node is sent and received individually with blocking directives. I don't have a large file at the moment but on a moderate-size acoustics problem run on 64 Frontera nodes the timing was:

 migration time:      4.14451 seconds 

This becomes increasingly expensive on large meshes.

Indirect communication and/or batching may be needed.

[stefanhenneking]

As an additional comment: at the moment, a lot of node data (except for middle nodes) are sent/received redundantly; there are various ways of avoiding this which should help with the performance of the data migration.