Create an in-memory key chain in ndvr simulation environment

[italovalcy]

Currently, the key chain is created in the default PIB/TPM ($HOME/.ndn/{ndnsec-key-file/,pib.db} and the router key/identity/certificate are created in the scenario context (not in the router context). As a consequence, we cannot run parallel simulations (due to file lock on $HOME/.ndn/pib.db) and also the n-th router knows about the identities of the (n-1)-th and not the opposite, which leads to inconsistency in Pib:

../src/ndnSIM/ndn-cxx/ndn-cxx/security/pib/pib.cpp:102: ndn::security::pib::Identity ndn::security::pib::Pib::getIdentity(const ndn::Name&) const: Assertion `m_identities.isConsistent()' failed.

NDVR should be able to run an in-memory router's KeyChain:

diff --git a/extensions/ndvr.cpp b/extensions/ndvr.cpp
index 0e01070..fede029 100644
--- a/extensions/ndvr.cpp
+++ b/extensions/ndvr.cpp
@@ -31,6 +31,7 @@ Ndvr::Ndvr(const ndn::security::SigningInfo& signingInfo, Name network, Name rou
   , m_rand(ns3::CreateObject<ns3::UniformRandomVariable>())
   , m_network(network)
   , m_routerName(routerName)
+  , m_keyChain("pib-memory:", "tpm-memory:", true)
   , m_helloIntervalIni(1)
   , m_helloIntervalCur(1)
   , m_helloIntervalMax(5)