[fix]: memory leak in mod_dyups when ssl session reuse is enabled.

[timebug]

Hi, @yzprofile

Issue #67 open for a long time, this problem also appeared in our online environment recently. I see @RocFang fix it just skip set_session and save_session function in PR #141, but after that will make proxy ssl session reuse not work.

So, I redesigned set_session and save_session function, the new solution will solve memory leak issue and keep ssl session reuse work:

1. Save the pointer address to ssl_session data in the upstream related variable, not pre request related variable such as dyups_ctx, avoid losing after request complete.
2. Use the upstream related memory pool to allocate ssl_session memory, it will destroy when free dynamic upstream or exit process.
3. Free up extra ssl session memory which allocate at OpenSSL immediately after ngx_ssl_set_session(pc->connection, ssl_session) success.

Steps to reproduce memory leak issue:

server {
    listen   8080;

    location / {
        proxy_ssl_verify        off;
        proxy_ssl_session_reuse on;

        proxy_pass https://$host;
        proxy_http_version 1.1;
        proxy_set_header Connection "";   
    }
}

server {
    listen 8088 ssl;
    ssl_certificate     ssl/test.crt;
    ssl_certificate_key ssl/test.key;

    location / {
        return 301;
    }
}

server {
    listen 8089 ssl;
    ssl_certificate     ssl/test.crt;
    ssl_certificate_key ssl/test.key;

    location / {
        return 302;
    }
}

server {
    listen 8081;
    location / {
        dyups_interface;
    }
}

case1: (keepalive or no keepalive)

1. curl -H "Host: dyhost" -d "server 127.0.0.1:8089;server 127.0.0.1:8088;" 127.0.0.1:8081/upstream/dyhost -v
2. for i in {1..10000}; do curl -H "host: dyhost" 127.0.0.1:8080 -v; d

case2: (keepalive or no keepalive)

1. for i in {1..10000}; do curl -H "Host: dyhost" -d "server 127.0.0.1:8089;server 127.0.0.1:8088;" 127.0.0.1:8081/upstream/dyhost -v; done
2. for i in {1..10000}; do curl -H "host: dyhost" 127.0.0.1:8080 -v; d

[timebug]

ping @chobits