[PATCH 00 of 11] [quic] reusing crypto contexts, and more #2

[Sergey Kandaurov]

Updated series to address arut@ comments:
- patches #1, #2, #4 unchanged
- patch #3 replaced with keys check in ngx_quic_ack_packet
- #5 updates ngx_quic_keys_cleanup and ngx_quic_compat_set_encryption_secret
- factored out common code for ngx_quic_crypto_open/seal
- ngx_quic_crypto_hp cleanup for CRYPTO_chacha_20
- new change to simpify ngx_quic_ciphers() API.
- new change to remove key field from ngx_quic_secret_t
- assorted fixes in using ngx_explicit_memzero