MEDIUM: guid: switch guid to more compact cebuis_tree

MEDIUM: guid: switch guid to more compact cebuis_tree

The current guid struct size is 56 bytes. Once reduced using compact
trees, it goes down to 32 (almost half). We're not on a critical path
and size matters here, so better switch to this.

It's worth noting that the name part could also be stored in the
guid_node at the end to save 8 extra byte (no pointer needed anymore),
however the purpose of this struct is to be embedded into other ones,
which is not compatible with having a dynamic size.

Affected struct sizes in bytes:

           Before     After   Diff
  server    4032       4032     0*
  proxy     3184       3160    -24
  listener   752        728    -24

*: struct server is full of holes and padding (176 bytes) and is
64-byte aligned. Moving the guid_node elsewhere such as after sess_conn
reduces it to 3968, or one less cache line. There's no point in moving
anything now because forthcoming patches will arrange other parts.