Rename types and variables used for packet number space.

Rename types and variables used for packet number space.

Quote: Conceptually, a packet number space is the context in which a packet
       can be processed and acknowledged.

ngx_quic_namespace_t => ngx_quic_send_ctx_t
qc->ns               => qc->send_ctx
ns->largest          => send_ctx->largest_ack

The ngx_quic_ns(level) macro now returns pointer, not just index:
    ngx_quic_get_send_ctx(c->quic, level)

ngx_quic_retransmit_ns() => ngx_quic_retransmit()
ngx_quic_output_ns() => ngx_quic_output_frames()