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/examples/consumer-with-timer.cpp b/examples/consumer-with-timer.cpp
index 6c7ef8c..83bb858 100644
--- a/examples/consumer-with-timer.cpp
+++ b/examples/consumer-with-timer.cpp
@@ -32,7 +32,7 @@
   
   ndn::Interest i(ndn::Name("/localhost/testApp/randomData"));
   i.setScope(1);
-  i.setInterestLifetime(1000);
+  i.setInterestLifetime(ndn::time::milliseconds(1000));
   i.setMustBeFresh(true);
 
   face.expressInterest(i,
@@ -50,7 +50,7 @@
     
     ndn::Interest i(ndn::Name("/localhost/testApp/randomData"));
     i.setScope(1);
-    i.setInterestLifetime(1000);
+    i.setInterestLifetime(ndn::time::seconds(1));
     i.setMustBeFresh(true);
 
     ndn::Face face(io);