QUIC: generate default stateless reset token key. Previously, if quic_stateless_reset_token_key was empty or unspecified, initial stateless reset token was not generated. However subsequent tokens were generated with empty key, which resulted in error with certain SSL libraries, for example OpenSSL. Now a random 32-byte stateless reset token key is generated if none is specified in the configuration. As a result, stateless reset tokens are now generated for all server ids.