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/security/key-chain.hpp b/src/security/key-chain.hpp
index 28642ae..e58cbf9 100644
--- a/src/security/key-chain.hpp
+++ b/src/security/key-chain.hpp
@@ -124,8 +124,8 @@
   shared_ptr<IdentityCertificate>
   prepareUnsignedIdentityCertificate(const Name& keyName,
                                      const Name& signingIdentity,
-                                     const MillisecondsSince1970& notBefore,
-                                     const MillisecondsSince1970& notAfter,
+                                     const time::system_clock::TimePoint& notBefore,
+                                     const time::system_clock::TimePoint& notAfter,
                                      const std::vector<CertificateSubjectDescription>& subjectDescription)
 
   {
@@ -360,8 +360,8 @@
     certificateName.append("KEY").append(keyName.get(-1)).append("ID-CERT").appendVersion();
 
     certificate->setName(certificateName);
-    certificate->setNotBefore(getNow());
-    certificate->setNotAfter(getNow() + 630720000 /* 20 years*/);
+    certificate->setNotBefore(time::system_clock::now());
+    certificate->setNotAfter(time::system_clock::now() + time::days(7300)/* ~20 years*/);
     certificate->setPublicKeyInfo(*pubKey);
     certificate->addSubjectDescription(CertificateSubjectDescription("2.5.4.41", keyName.toUri()));
     certificate->encode();