[pmix server] Simplify pmix_server_process_peer
[Compile - OK, Runtime - OK]
[pmix server] Fix pmix_server_process_peer cleanup code.
[pmix server] Refactoring of pmix_peer_process.c
1. Improve PMIX_ADD_KP
2. Extract PMI_CMD_GET handling code from the long
pmix_server_process_peer function and split it onto several logically
complete parts.