Make PrimitiveSerialDescriptor.hashCode() stable

[vgv]

Describe the bug
PrimitiveSerialDescriptor class doesn't have custom hashCode() implemented, so, it can change between different JVMs vendors, versions, servers, between different app run etc.
Plus, we can change hashCode generation strategy (-XX:hashCode JVM option) – this will change hashCode value too.

I think it is good idea to add stable hashCode() implementation to PrimitiveSerialDescriptor