REORG: quic: remove quic_cid_trees reference from proto_quic

REORG: quic: remove quic_cid_trees reference from proto_quic

Previous commit removed access/manipulation to QUIC CID global tree
outside of quic_cid module. This ensures that proper locking is always
performed.

This commit finalizes this cleanup by marking CID global tree as static
only to quic_cid source file. Initialization of this tree is removed
from proto_quic and now performed using dedicated initcalls
quic_alloc_global_cid_tree().

As a side change, complete CID global tree documentation, in particular
to explain CID global tree artificial splitting and ODCID handling.
Overall, the code is now clearer and safer.