Upstream: optimized use of SSL contexts (ticket #1234). To ensure optimal use of memory, SSL contexts for proxying are now inherited from previous levels as long as relevant proxy_ssl_* directives are not redefined. Further, when no proxy_ssl_* directives are redefined in a server block, we now preserve plcf->upstream.ssl in the "http" section configuration to inherit it to all servers. Similar changes made in uwsgi, grpc, and stream proxy.