[pmix server] Move all peer management code to platform-dependent part
of pmix. [Compile OK, Runtime -].
[pmix server] Complete migration of peer management code to "platform"
NOTE: this commit spawns WARRNINGS: "ISO C forbids conversion of
function pointer to object pointer type [-Wpedantic]"
This will be fixed in later commits.
[Compile OK, Runtime -].
[pmix server] Fix bug with using OBJ_DESTRUCT instead of OBJ_RELEASE.
[Compile OK, Runtime OK].