Add internal ref set_name fn instead of realloc

Add internal ref set_name fn instead of realloc

The refdb_fs implementation calls realloc directly on a reference
object when it wants to rename it.  It is not a public object, so
this doesn't mess with the immutability of references, but it does
assume certain constraints on the reference representation.  This
commit wraps that assumption in an isolated API to isolate it.