face: move packet encoding to Impl class

Practical limit of packet size is now enforced on encoded
NDNLPv2 packets instead of network-layer packets.

If a packet exceeds size limit, the face throws a
Face::OversizedPacketError exception. Since packet encoding may
be asynchronous, this exception could be thrown by
Face::processEvents instead of Face::expressInterest and
Face::put.

refs #4228

Change-Id: Ib68cf80b3b8967fdd0ba040bd8ee595a0eff740e
diff --git a/tests/integrated/face.cpp b/tests/integrated/face.cpp
index 038b499..cf7c872 100644
--- a/tests/integrated/face.cpp
+++ b/tests/integrated/face.cpp
@@ -150,9 +150,7 @@
                        bind(&FacesFixture::onData, this),
                        bind(&FacesFixture::onNack, this),
                        bind(&FacesFixture::onTimeout, this));
-
-  BOOST_REQUIRE_NO_THROW(face.processEvents());
-
+  face.processEvents();
   BOOST_CHECK_EQUAL(nData, 1);
   BOOST_CHECK_EQUAL(nNacks, 0);
   BOOST_CHECK_EQUAL(nTimeouts, 0);
@@ -162,25 +160,29 @@
                        bind(&FacesFixture::onData, this),
                        bind(&FacesFixture::onNack, this),
                        bind(&FacesFixture::onTimeout, this));
+  face.processEvents();
+  BOOST_CHECK_EQUAL(nData, 1);
+  BOOST_CHECK_EQUAL(nNacks, 0);
+  BOOST_CHECK_EQUAL(nTimeouts, 1);
 
   Name veryLongName;
   for (size_t i = 0; i <= MAX_NDN_PACKET_SIZE / 10; i++) {
     veryLongName.append("0123456789");
   }
 
-  BOOST_CHECK_THROW(face.expressInterest(Interest(veryLongName), nullptr, nullptr, nullptr),
-                    Face::Error);
+  BOOST_CHECK_THROW(do {
+    face.expressInterest(Interest(veryLongName), nullptr, nullptr, nullptr);
+    face.processEvents();
+  } while (false), Face::OversizedPacketError);
 
   shared_ptr<Data> data = make_shared<Data>(veryLongName);
   data->setContent(reinterpret_cast<const uint8_t*>("01234567890"), 10);
   m_keyChain.sign(*data);
-  BOOST_CHECK_THROW(face.put(*data), Face::Error);
+  BOOST_CHECK_THROW(do {
+    face.put(*data);
+    face.processEvents();
+  } while (false), Face::OversizedPacketError);
 
-  BOOST_REQUIRE_NO_THROW(face.processEvents());
-
-  BOOST_CHECK_EQUAL(nData, 1);
-  BOOST_CHECK_EQUAL(nNacks, 0);
-  BOOST_CHECK_EQUAL(nTimeouts, 1);
 }
 
 BOOST_AUTO_TEST_CASE(Tcp)