session CHANGE use monotonic clock where possible

Fixes #43
diff --git a/src/io.c b/src/io.c
index f2f8084..30f7a79 100644
--- a/src/io.c
+++ b/src/io.c
@@ -52,7 +52,7 @@
         return 0;
     }
 
-    nc_gettimespec(&ts_inact_timeout);
+    nc_gettimespec_mono(&ts_inact_timeout);
     nc_addtimespec(&ts_inact_timeout, inact_timeout);
     do {
         switch (session->ti_type) {
@@ -136,7 +136,7 @@
         if (r == 0) {
             /* nothing read */
             usleep(NC_TIMEOUT_STEP);
-            nc_gettimespec(&ts_cur);
+            nc_gettimespec_mono(&ts_cur);
             if ((nc_difftimespec(&ts_cur, &ts_inact_timeout) < 1) || (nc_difftimespec(&ts_cur, ts_act_timeout) < 1)) {
                 if (nc_difftimespec(&ts_cur, &ts_inact_timeout) < 1) {
                     ERR("Session %u: inactive read timeout elapsed.", session->id);
@@ -152,7 +152,7 @@
             readd += r;
 
             /* reset inactive timeout */
-            nc_gettimespec(&ts_inact_timeout);
+            nc_gettimespec_mono(&ts_inact_timeout);
             nc_addtimespec(&ts_inact_timeout, inact_timeout);
         }
 
@@ -282,7 +282,7 @@
         return NC_MSG_ERROR;
     }
 
-    nc_gettimespec(&ts_act_timeout);
+    nc_gettimespec_mono(&ts_act_timeout);
     nc_addtimespec(&ts_act_timeout, NC_READ_ACT_TIMEOUT * 1000);
 
     /* read the message */