CL/HIER: Add allgatherv

[nsarka]

This PR adds CL/HIER allgatherv and updates the allgatherv gtest to test non-contiguous dst buffers. It is meant to be used in conjunction with my TL/SHM gatherv implementation.

The algorithm is:

• Node-level gatherv, all sendbufs on the node are put into a scratch buffer
• Leader-level allgatherv on the scratch buffer, the result is put into the dst buffer
• Node-level in-place bcast on the dst buffer
• If the buffer is non-contig, unpack by memcpying the packed buffer into the right displacements

Comparison Data

2 nodes, 32 PPN on lego-grace cg (1 socket, 72 cores/socket)

Size	UCC after PR (cl_hier + tl_shm + tl_ucp)	UCC before PR (cl_basic + tl_ucp)	HCOLL	%-Gain, After vs. HCOLL	%-Gain, Old vs. HCOLL	%-Gain, Before vs. After
1	7.37	19.88	7.41	0.54%	-168.29%	62.93%
2	7.81	19.75	7.47	-4.55%	-164.39%	60.46%
4	7.62	19.79	7.65	0.39%	-158.69%	61.50%
8	8.35	19.78	8.13	-2.71%	-143.30%	57.79%
16	9.49	19.8	9.44	-0.53%	-109.75%	52.07%
32	9.34	22.91	9.67	3.41%	-136.92%	59.23%
64	9.17	22.96	10.03	8.57%	-128.91%	60.06%
128	10.47	27.55	10.88	3.77%	-153.22%	62.00%
256	12.68	33.37	12.46	-1.77%	-167.82%	62.00%
512	19.57	180	25.31	22.68%	-611.18%	89.13%
1024	25.38	189.21	308.33	91.77%	38.63%	86.59%
2048	38.86	198.27	311.55	87.53%	36.36%	80.40%

[swx-jenkins3]

Can one of the admins verify this patch?

[manjugv]

@aamirshafi Please review