ConnectionManager: allow to provide RNG through configuration

Change-Id: Ic24123e329b221f5f1e24d3df6f3cc93649c4591
diff --git a/src/connectionmanager.cpp b/src/connectionmanager.cpp
index 87704a8..2e0e99b 100644
--- a/src/connectionmanager.cpp
+++ b/src/connectionmanager.cpp
@@ -381,7 +381,7 @@
 public:
     explicit Impl(std::shared_ptr<ConnectionManager::Config> config_)
         : config_ {std::move(createConfig(config_))}
-        , rand_ {dht::crypto::getSeededRandomEngine<std::mt19937_64>()}
+        , rand_ {config_->rng ? *config_->rng : dht::crypto::getSeededRandomEngine<std::mt19937_64>()}
     {
         loadTreatedMessages();
         if(!config_->ioContext) {