mpid: add MPIR_Data and MPID_{Send,Recv}_data (WIP)

[hzhou]

Pull Request Description

Add new MPID send/recv interfaces that accept a data pointer rather
than the (buf, count, datatype) triplet. This allows carrying more
information for extensions. For example, we can support data chunks that
often occur in pipelining algorithms. We may also extend MPIR_Data to
add memory attributes.

Replace -

MPID_Send(buf, count, datatype, ...);
MPID_Recv(buf, count, datatype, ...);

with

MPID_Send_data(MPIR_Data *data, ...);
MPID_Recv_data(MPIR_Data *data, ...);

, where MPIR_Data is defined as

typedef struct MPIR_Data {
    void *buf;
    MPI_Aint count;
    MPI_Datatype datatype;
    MPI_Aint offset;
    MPI_Aint length;
} MPIR_Data;

This allows MPIR-layer pipelining-like algorithms to break the data into chunks without worrying about creating new chunk datatypes.

Author Checklist

• Provide Description
  Particularly focus on why, not what. Reference background, issues, test failures, xfail entries, etc.
• Commits Follow Good Practice
  Commits are self-contained and do not do two things at once.
  Commit message is of the form: module: short description
  Commit message explains what's in the commit.
• Passes All Tests
  Whitespace checker. Warnings test. Additional tests via comments.
• Contribution Agreement
  For non-Argonne authors, check contribution agreement.
  If necessary, request an explicit comment from your companies PR approval manager.