Use uint64_t for uptime in millis, not int64_t

docs/README.md

@@ -134,7 +134,7 @@ to an event handler:
 enum {
   MG_EV_ERROR,       // Error                        char *error_message
   MG_EV_OPEN,        // Connection created           NULL
-  MG_EV_POLL,        // mg_mgr_poll iteration        int64_t *milliseconds
+  MG_EV_POLL,        // mg_mgr_poll iteration        uint64_t *milliseconds
   MG_EV_RESOLVE,     // Host name is resolved        NULL
   MG_EV_CONNECT,     // Connection established       NULL
   MG_EV_ACCEPT,      // Connection accepted          NULL
@@ -149,7 +149,7 @@ enum {
   MG_EV_MQTT_CMD,    // MQTT low-level command       struct mg_mqtt_message *
   MG_EV_MQTT_MSG,    // MQTT PUBLISH received        struct mg_mqtt_message *
   MG_EV_MQTT_OPEN,   // MQTT CONNACK received        int *connack_status_code
-  MG_EV_SNTP_TIME,   // SNTP time received           int64_t *milliseconds
+  MG_EV_SNTP_TIME,   // SNTP time received           uint64_t *milliseconds
   MG_EV_USER,        // Starting ID for user events
 };
 ```
@@ -1839,8 +1839,8 @@ mg_tls_init(c, &opts);
 
 ```c
 struct mg_timer {
-  int64_t period_ms;        // Timer period in milliseconds
-  int64_t expire;           // Expiration timestamp in milliseconds
+  uint64_t period_ms;       // Timer period in milliseconds
+  uint64_t expire;          // Expiration timestamp in milliseconds
   unsigned flags;           // Possible flags values below
 #define MG_TIMER_REPEAT 1   // Call function periodically, otherwise run once
 #define MG_TIMER_RUN_NOW 2  // Call immediately when timer is set
@@ -1856,7 +1856,7 @@ as the `mg_mgr_poll()` timeout argument in the main event loop.
 ### mg\_timer\_init()
 
 ```c
-void mg_timer_init(struct mg_timer *t, int64_t period_ms, unsigned flags,
+void mg_timer_init(struct mg_timer *t, uint64_t period_ms, unsigned flags,
                    void (*fn)(void *), void *fn_data);
 ```
 
@@ -1904,7 +1904,7 @@ mg_timer_free(&timer);
 ### mg\_timer\_poll()
 
 ```c
-void mg_timer_poll(int64_t uptime_ms);
+void mg_timer_poll(uint64_t uptime_ms);
 ```
 
 Traverse list of timers and call them if current timestamp `uptime_ms` is
@@ -1921,8 +1921,7 @@ Return value: None
 Usage example:
 
 ```c
-int64_t now = mg_millis();
-mg_timer_poll(now);
+mg_timer_poll(mg_millis());
 ```
 
 ## Time