tools/ceph-dencoder: register dencoders in plugin

tools/ceph-dencoder: register dencoders in plugin

so we can allocate and deallocate dencoders in the shared library,
instead of allocating them in the shared library, while deallocating
them in the executable.

after this change

- the plugin holds the strong references of the dencoders
- the registry holds the plugins and weak references of dencoders
- the dencoder shared libraries calls the method exposed by plugin
  to alloc/dealloc the dencoders

this change should address the segfault when compiling with Clang.

Signed-off-by: Kefu Chai <kchai@redhat.com>