v0.0.18

Sync with upstream master (25 commits since v0.0.17).

Key changes:

• feature: add ffi api ngx_stream_lua_ffi_socket_tcp_getfd (fixes lua-resty-core 0.1.32+ compatibility)
• feature: add socket options keepintvl and keepcnt for tcp
• feature: implement serversslhandshake method on downstream sockets
• feature: add ngx_stream_lua_ffi_get_upstream_ssl_pointer
• feature: proxy_ssl_certificate_by_lua directives
• feature: add new API: tcpsock:get_ssl_session
• bugfix: fix the compatibility issues with freenginx
• bugfix: didn't close cosocket when nginx shutdown timer has been triggered
• bugfix: prevent uthread crash by checking coroutine reference before deletion