Release 1.18.0

- [API Change] Can specify clock granularity in engine settings.
- [BUGFIX] Pacer uses fixed clock granularity.  Since the change on
  2018-04-09, it is not necessary to try to approximate the next tick
  time in the pacer: it can use fix clock granularity specified by
  the user.
- [BUGFIX] Do not tick constantly before handshake is done.
- [BUGFIX] Do not exit tick prematurely: reuse packet with ACK.  Even
  if we cannot allocate *more* packets, we could still be able to write
  to one already allocated.
- [BUGFIX] Do not schedule pacer if there are no lost packets.

include/lsquic.h

@@ -24,8 +24,8 @@ extern "C" {
 #endif
 
 #define LSQUIC_MAJOR_VERSION 1
-#define LSQUIC_MINOR_VERSION 17
-#define LSQUIC_PATCH_VERSION 15
+#define LSQUIC_MINOR_VERSION 18
+#define LSQUIC_PATCH_VERSION 0
 
 /**
  * Engine flags:
@@ -245,6 +245,9 @@ struct lsquic_stream_if {
 /** By default, packets are paced */
 #define LSQUIC_DF_PACE_PACKETS      1
 
+/** Default clock granularity is 1000 microseconds */
+#define LSQUIC_DF_CLOCK_GRANULARITY      1000
+
 struct lsquic_engine_settings {
     /**
      * This is a bit mask wherein each bit corresponds to a value in
@@ -432,6 +435,11 @@ struct lsquic_engine_settings {
      */
     int             es_pace_packets;
 
+    /**
+     * Clock granularity information is used by the pacer.  The value
+     * is in microseconds; default is @ref LSQUIC_DF_CLOCK_GRANULARITY.
+     */
+    unsigned        es_clock_granularity;
 };
 
 /* Initialize `settings' to default values */