Changed time_t to uint64_t for serialization

time_t is implementation-, architecture-, and apparently
compiler-dependent.  As an example, on my machine if I build a 64-bit
binary, sizeof(time_t) is 8, but for a 32-bit binary it's 4.  uint64_t
is therefore much more consistent for serialization, given that RPC
calls are potentially made between different machines.

src/cryptonote_protocol/cryptonote_protocol_defs.h

@@ -27,14 +27,14 @@ namespace cryptonote
     std::string peer_id;
 
     uint64_t recv_count;
-    time_t recv_idle_time;
+    uint64_t recv_idle_time;
 
     uint64_t send_count;
-    time_t send_idle_time;
+    uint64_t send_idle_time;
 
     std::string state;
 
-    time_t live_time;
+    uint64_t live_time;
 
     BEGIN_KV_SERIALIZE_MAP()
       KV_SERIALIZE(incoming)