SPARK-3357 [CORE] Internal log messages should be set at DEBUG level instead of INFO

Demote some 'noisy' log messages to debug level. I added a few more, to include everything that gets logged in stanzas like this:

```
15/03/01 00:03:54 INFO BlockManager: Removing broadcast 0
15/03/01 00:03:54 INFO BlockManager: Removing block broadcast_0_piece0
15/03/01 00:03:54 INFO MemoryStore: Block broadcast_0_piece0 of size 839 dropped from memory (free 277976091)
15/03/01 00:03:54 INFO BlockManagerInfo: Removed broadcast_0_piece0 on localhost:49524 in memory (size: 839.0 B, free: 265.1 MB)
15/03/01 00:03:54 INFO BlockManagerMaster: Updated info of block broadcast_0_piece0
15/03/01 00:03:54 INFO BlockManager: Removing block broadcast_0
15/03/01 00:03:54 INFO MemoryStore: Block broadcast_0 of size 1088 dropped from memory (free 277977179)
15/03/01 00:03:54 INFO ContextCleaner: Cleaned broadcast 0
```

as well as regular messages like

```
15/03/01 00:02:33 INFO MemoryStore: ensureFreeSpace(2640) called with curMem=47322, maxMem=278019440
```

WDYT? good or should some be left alone?

CC mengxr who suggested some of this.

Author: Sean Owen <sowen@cloudera.com>

Closes #4838 from srowen/SPARK-3357 and squashes the following commits:

dce75c1 [Sean Owen] Back out some debug level changes
d9b784d [Sean Owen] Demote some 'noisy' log messages to debug level

core/src/main/scala/org/apache/spark/ContextCleaner.scala

@@ -188,10 +188,10 @@ private[spark] class ContextCleaner(sc: SparkContext) extends Logging {
   /** Perform broadcast cleanup. */
   def doCleanupBroadcast(broadcastId: Long, blocking: Boolean) {
     try {
-      logDebug("Cleaning broadcast " + broadcastId)
+      logDebug(s"Cleaning broadcast $broadcastId")
       broadcastManager.unbroadcast(broadcastId, true, blocking)
       listeners.foreach(_.broadcastCleaned(broadcastId))
-      logInfo("Cleaned broadcast " + broadcastId)
+      logDebug(s"Cleaned broadcast $broadcastId")
     } catch {
       case e: Exception => logError("Error cleaning broadcast " + broadcastId, e)
     }

core/src/main/scala/org/apache/spark/storage/BlockManager.scala

@@ -1074,7 +1074,7 @@ private[spark] class BlockManager(
    * Remove all blocks belonging to the given broadcast.
    */
   def removeBroadcast(broadcastId: Long, tellMaster: Boolean): Int = {
-    logInfo(s"Removing broadcast $broadcastId")
+    logDebug(s"Removing broadcast $broadcastId")
     val blocksToRemove = blockInfo.keys.collect {
       case bid @ BroadcastBlockId(`broadcastId`, _) => bid
     }
@@ -1086,7 +1086,7 @@ private[spark] class BlockManager(
    * Remove a block from both memory and disk.
    */
   def removeBlock(blockId: BlockId, tellMaster: Boolean = true): Unit = {
-    logInfo(s"Removing block $blockId")
+    logDebug(s"Removing block $blockId")
     val info = blockInfo.get(blockId).orNull
     if (info != null) {
       info.synchronized {

core/src/main/scala/org/apache/spark/storage/BlockManagerMaster.scala

@@ -61,7 +61,7 @@ class BlockManagerMaster(
       tachyonSize: Long): Boolean = {
     val res = askDriverWithReply[Boolean](
       UpdateBlockInfo(blockManagerId, blockId, storageLevel, memSize, diskSize, tachyonSize))
-    logInfo("Updated info of block " + blockId)
+    logDebug(s"Updated info of block $blockId")
     res
   }
 

core/src/main/scala/org/apache/spark/storage/MemoryStore.scala

@@ -184,7 +184,7 @@ private[spark] class MemoryStore(blockManager: BlockManager, maxMemory: Long)
       val entry = entries.remove(blockId)
       if (entry != null) {
         currentMemory -= entry.size
-        logInfo(s"Block $blockId of size ${entry.size} dropped from memory (free $freeMemory)")
+        logDebug(s"Block $blockId of size ${entry.size} dropped from memory (free $freeMemory)")
         true
       } else {
         false