Use libuv's cached current time for `time_now()`

We are calling time_now() a huge amount, and it is a major consumer of
CPU cycles, but we don't need it: most of the time the current event
loop time is enough.

llarp/ev/ev.hpp

@@ -75,11 +75,11 @@ namespace llarp
     virtual bool
     running() const = 0;
 
+    // Returns a current steady clock time value representing the current time with event loop tick
+    // granularity.  That is, the value is typically only updated at the beginning of an event loop
+    // tick.
     virtual llarp_time_t
-    time_now() const
-    {
-      return llarp::time_now_ms();
-    }
+    time_now() const = 0;
 
     // Calls a function/lambda/etc.  If invoked from within the event loop itself this calls the
     // given lambda immediately; otherwise it passes it to `call_soon()` to be queued to run at the

llarp/ev/ev_libuv.hpp

@@ -31,6 +31,12 @@ namespace llarp::uv
     bool
     running() const override;
 
+    llarp_time_t
+    time_now() const override
+    {
+      return m_Impl->now();
+    }
+
     void
     call_later(llarp_time_t delay_ms, std::function<void(void)> callback) override;