Keep MPI tags in Connector algorithm classes constant--WIP

[nselliott]

MappingConnectorAlgorithm and OverlapConnectorAlgorithm used a constantly increasing static value for the MPI tags used in sends and receives, so that each invocation used a unique value. This may have been necessary in the past, but is not now, since we have strict error-checking to ensure that there are no stray messages lingering for any tag value. The increasing values risk surpassing upper bounds for valid values of the MPI tags.

WIP, do not merge.