MINOR: quic: support multiple random CID generation for BE side

MINOR: quic: support multiple random CID generation for BE side

When a new backend connection is instantiated, a CID is first randomly
generated. It will serve as the first DCID for incoming packets from the
server. Prior to this patch, if the generated CID caused a collision
with an other entries from another connection, an error is reported and
the connection cannot be allocated.

This patch improves this procedure by implementing retries when a
collision occurs. Now, at most three attemps will be performed before
giving up. This is the same procedure already performed for CIDs
instantiated after RETIRE_CONNECTION_ID frame parsing.

Along with this functional change, qc_new_conn() is refactored for
backend instantiation. The CID generation is extracted from it and the
value is passed as an argument. This is considered cleaner as the code
is more similar between frontend and backend sides.