SemMedDB V43 update

[erikyao]

This PR update the SemMedDB API per https://github.com/biothings/pending.api/issues/63.

It also fixes the following issues towards piped CUIs, retired CUI, and equivalent NCBIGene IDs:

• https://github.com/biothings/semmeddb/issues/2
• https://github.com/biothings/semmeddb/issues/3
• https://github.com/biothings/semmeddb/issues/5
• https://github.com/biothings/semmeddb/issues/6
• https://github.com/biothings/semmeddb/issues/7