Swtich from vectors to unordered_maps in remote neighbor update code

[iljah]

Updating copies of remote neighbors' data is too slow for some applications because the code relies hevily on unordered_maps which in practice are slow to access. MPI requests and other transient MPI data should use vectors instead to increase performance.

[iljah]

So many internal changes in last 5 years that this one is probably not applicable anymore or if it is then I'll create another issue after profiling latest code...