Fixes #3793: Move r29243 to v1.7 branch (prevent infinite loop when pml modex not found)

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r29243 [[BR]]
Ensure we don't enter into an infinite loop looking for the PML modex key if it isn't present. The PMI implementation will load ALL modex keys when the first key is queried, so the hash db component can safely return "not found" if a subsequent key isn't present. The PML modex_recv needs to assume everything is okay if the modex recv fails to return a value.

cmr:v1.7.3:reviewer=jladd:subject=Prevent infinite loop when PML modex not found
Fixes #3797: Move r29248 to v1.7 branch (Do not use modex recv)

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r29248 [[BR]]
Do not use modex recv to fetch the locality as this will automatically force retrieval of hostnames, which we are trying to avoid. Instead, use the database API to fetch that info.

cmr:v1.7.3:reviewer=hjelmn