QUIC: refactored key handling. All key handling functionality is moved into ngx_quic_protection.c. Public structures from ngx_quic_protection.h are now private and new methods are available to manipulate keys. A negotiated cipher is cached in QUIC connection from the set secret callback to avoid calling SSL_get_current_cipher() on each encrypt/decrypt operation. This also reduces the number of unwanted c->ssl->connection occurrences.