Refactor mg polling

* Change return type of mg_mgr_poll to return number of events
 * Add mg_mgr_min_timer
 * Refactor main poll loop to remove LwIP-specific stuff

CL: Refactor mg polling

PUBLISHED_FROM=dc94618b32fa3c84a2f053bd04d134297780ec82

docs/c-api/mg_net.h/struct_mg_mgr.md

@@ -13,6 +13,7 @@ signature: |
   #endif
     void *user_data; /* User data */
     int num_ifaces;
+    int num_calls;
     struct mg_iface **ifaces; /* network interfaces */
     const char *nameserver;   /* DNS server to use */
   };