API changes

• SOCKS5 client and server support
• Timer value is no longer cleared after MG_EV_TIMER event if it was not changed. User's handler must set it to 0 explicitly to avoid repeated invocation.

FIxes

• SSL/TLS listener fix for TI SimpleLink networking interface
• Double-counting of recved bytes in LWIP interface caused issues
• MQTT SUBSCRIBE command parsing in the broker code could cause crashes
• FIxed CGI script connection lifetime management