all: Refactoring work with time using boost::chrono

Now the library has two clocks: time::steady_clock and
time::system_clock, following (boost|std)::chrono.  In addition to
standard now() method, the library contains several helpers to convert
to/from UnixTimestamp (microsecond resolution) and IsoString (optional
microsecond resolution).  The IsoString conversions use
boost::posix_time routines, since boost::chrono supports extended IO
support only starting boost version 1.52 (Boost Chrono V2).

This commit breaks compatibility with previous API.  All time-related
Data/Interest calls must explicitly use time units to specify
FreshnessPeriod/InterestLifetime.

Brief usage/conversion guide:

- creation of time units does not support double/float types.  If
  necessary to create time unit from double, ``ndn::duration<double>`` (for
  seconds) needs to be used instead.  In some cases, this would also
  require ``ndn::duration_cast``, if the target is not ``ndn::nanoseconds``.
- ndn::getNow, ndn::ndn_getNowMilliseconds, ndn::time::now are all
  removed in favor of the now() method in a specific clock:

    * time::system_clock::now();
    * time::steady_clock::now();

- When necessary to convert system_clock::TimePoint to unix timestamp,
  ``time::toUnixTimestamp`` can be used.  This method return number of
  milliseconds since UNIX epoch as ``ndn::time::milliseconds`` type.
  Use count() method to obtain number as an integral value.

Change-Id: Icd688bc6766e008d60c3d2888173627874526e47
Refs: #1152
diff --git a/src/util/scheduler.cpp b/src/util/scheduler.cpp
index 388d5e7..377800f 100644
--- a/src/util/scheduler.cpp
+++ b/src/util/scheduler.cpp
@@ -47,16 +47,16 @@
   bool m_isValid;
 };
 
-Scheduler::EventInfo::EventInfo(const time::Duration& after,
-                        const time::Duration& period,
-                        const Event& event)
-  : m_scheduledTime(time::now() + after)
+Scheduler::EventInfo::EventInfo(const time::nanoseconds& after,
+                                const time::nanoseconds& period,
+                                const Event& event)
+  : m_scheduledTime(time::steady_clock::now() + after)
   , m_period(period)
   , m_event(event)
 {
 }
 
-Scheduler::EventInfo::EventInfo(const time::Point& when, const EventInfo& previousEvent)
+Scheduler::EventInfo::EventInfo(const time::steady_clock::TimePoint& when, const EventInfo& previousEvent)
   : m_scheduledTime(when)
   , m_period(previousEvent.m_period)
   , m_event(previousEvent.m_event)
@@ -64,10 +64,10 @@
 {
 }
 
-time::Duration
+time::nanoseconds
 Scheduler::EventInfo::expiresFromNow() const
 {
-  time::Point now = time::now();
+  time::steady_clock::TimePoint now = time::steady_clock::now();
   if (now > m_scheduledTime)
     return time::seconds(0); // event should be scheduled ASAP
   else
@@ -84,15 +84,15 @@
 }
 
 EventId
-Scheduler::scheduleEvent(const time::Duration& after,
+Scheduler::scheduleEvent(const time::nanoseconds& after,
                          const Event& event)
 {
   return schedulePeriodicEvent(after, time::nanoseconds(-1), event);
 }
 
 EventId
-Scheduler::schedulePeriodicEvent(const time::Duration& after,
-                                 const time::Duration& period,
+Scheduler::schedulePeriodicEvent(const time::nanoseconds& after,
+                                 const time::nanoseconds& period,
                                  const Event& event)
 {
   EventQueue::iterator i = m_events.insert(EventInfo(after, period, event));
@@ -154,13 +154,13 @@
   m_isEventExecuting = true;
 
   // process all expired events
-  time::Point now = time::now();
+  time::steady_clock::TimePoint now = time::steady_clock::now();
   while(!m_events.empty() && m_events.begin()->m_scheduledTime <= now)
     {
       EventQueue::iterator head = m_events.begin();
       
       Event event = head->m_event;
-      if (head->m_period < 0)
+      if (head->m_period < time::nanoseconds::zero())
         {
           head->m_eventId->invalidate();
           m_events.erase(head);