coll/han: alltoallv bugfixes

Fix several bugs in the original implementation:
 - Fallback if transform is in-place
 - Fix bug related to empty messages
 - Introduce barrier before exit during error
 - Handle datatypes with negative lower bounds
 - Fix a completion-ordering bug
 - Cleanup some unused variables

Signed-off-by: Luke Robison <lrbison@amazon.com>

Author: Luke Robison