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