Cleanup the temporary memory allocation in collectives

[bosilca]

Cleanup the memory handling for temporary buffers in some of the collective modules. Added a new function opal_datatype_span, to compute the memory span of count number of datatype, excluding the gaps in the beginning and at the end. If a memory allocation is made using the returned value, the gap (also returned) should be removed from the allocated pointer.

[ggouaillardet]

what about using a new function ?

int opal_datatype_alloc(opal_datatype_t * type, int count, void ** buf, void ** buf_free);

the rationale is code factorization, plus most of the time, we only need true_extent for

buf = buf_free - true_extent

[bosilca]

The difference with what is already inside is marginal (yours allocate and returns memory, mine if returning everything you need to do it right yourself). I prefer not to have a memory allocator down in OPAL datatype, but instead let people handle this the way they want/need.

[jsquyres]

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