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/encoding/cryptopp/asn_ext.hpp b/src/encoding/cryptopp/asn_ext.hpp
index e0a9a6b..b02b3e9 100644
--- a/src/encoding/cryptopp/asn_ext.hpp
+++ b/src/encoding/cryptopp/asn_ext.hpp
@@ -16,14 +16,14 @@
 namespace ndn {
 
 namespace Asn {
-struct Error : public std::runtime_error { Error(const std::string &what) : std::runtime_error(what) {} };
+struct Error : public std::runtime_error { Error(const std::string& what) : std::runtime_error(what) {} };
 }
 
 size_t
-DEREncodeGeneralTime(CryptoPP::BufferedTransformation &bt, MillisecondsSince1970 time);
+DEREncodeGeneralTime(CryptoPP::BufferedTransformation& bt, const time::system_clock::TimePoint& time);
 
 void
-BERDecodeTime(CryptoPP::BufferedTransformation &bt, MillisecondsSince1970 &time);
+BERDecodeTime(CryptoPP::BufferedTransformation& bt, time::system_clock::TimePoint& time);
 
 } // namespace ndn