Increased level of some verbose logs from master and agent.

Increases vlog level of some recurring events to improve readability.
Specifically, we now log the following events on vlog level two
instead of level one:
  "Sending heartbeat",
  "Received ping from <slave-observer>",
  "Querying resource estimator for oversubscribable resources",
  "Received oversubscribable resources".

Review: https://reviews.apache.org/r/62887/

src/master/master.hpp

@@ -346,7 +346,7 @@ class Heartbeater : public process::Process<Heartbeater<Message, Event>>
   {
     // Only send a heartbeat if the connection is not closed.
     if (http.closed().isPending()) {
-      VLOG(1) << "Sending heartbeat to " << logMessage;
+      VLOG(2) << "Sending heartbeat to " << logMessage;
 
       Message message(heartbeatMessage);
       http.send<Message, Event>(message);

src/slave/slave.cpp

@@ -4963,7 +4963,7 @@ void Slave::executorMessage(
 // within that (more generous) timeout.
 void Slave::ping(const UPID& from, bool connected)
 {
-  VLOG(1) << "Received ping from " << from;
+  VLOG(2) << "Received ping from " << from;
 
   if (!connected && state == RUNNING) {
     // This could happen if there is a one-way partition between
@@ -6489,7 +6489,7 @@ Future<Nothing> Slave::garbageCollect(const string& path)
 
 void Slave::forwardOversubscribed()
 {
-  VLOG(1) << "Querying resource estimator for oversubscribable resources";
+  VLOG(2) << "Querying resource estimator for oversubscribable resources";
 
   resourceEstimator->oversubscribable()
     .onAny(defer(self(), &Self::_forwardOversubscribed, lambda::_1));
@@ -6503,7 +6503,7 @@ void Slave::_forwardOversubscribed(const Future<Resources>& oversubscribable)
                << (oversubscribable.isFailed()
                    ? oversubscribable.failure() : "future discarded");
   } else {
-    VLOG(1) << "Received oversubscribable resources "
+    VLOG(2) << "Received oversubscribable resources "
             << oversubscribable.get() << " from the resource estimator";
 
     // Oversubscribable resources must be tagged as revocable.