BUG/MEDIUM: quic: QUIC CID removed from tree without locking

BUG/MEDIUM: quic: QUIC CID removed from tree without locking

This bug arrived with this commit:

   BUG/MINOR: quic: Wrong RETIRE_CONNECTION_ID sequence number chec

Every connection ID manipulations against the by thread trees used to store the
connection IDs must be done under the trees locks. These trees are accessed by
the low level connection identification code.

When receiving a RETIRE_CONNECTION_ID frame, the concerned connection ID must
be deleted from the its underlying by thread tree but not without locking!
Add a WR lock around ebmb_delete() call to do so.

Must be backported as far as 2.7.