[SPARK-3958] TorrentBroadcast cleanup / debugging improvements.

This PR makes several changes to TorrentBroadcast in order to make
it easier to reason about, which should help when debugging SPARK-3958.
The key changes:

- Remove all state from the global TorrentBroadcast object.  This state
  consisted mainly of configuration options, like the block size and
  compression codec, and was read by the blockify / unblockify methods.
  Unfortunately, the use of `lazy val` for `BLOCK_SIZE` meant that the block
  size was always determined by the first SparkConf that TorrentBroadast was
  initialized with; as a result, unit tests could not properly test
  TorrentBroadcast with different block sizes.

  Instead, blockifyObject and unBlockifyObject now accept compression codecs
  and blockSizes as arguments.  These arguments are supplied at the call sites
  inside of TorrentBroadcast instances.  Each TorrentBroadcast instance
  determines these values from SparkEnv's SparkConf.  I was careful to ensure
  that we do not accidentally serialize CompressionCodec or SparkConf objects
  as part of the TorrentBroadcast object.

- Remove special-case handling of local-mode in TorrentBroadcast.  I don't
  think that broadcast implementations should know about whether we're running
  in local mode.  If we want to optimize the performance of broadcast in local
  mode, then we should detect this at a higher level and use a dummy
  LocalBroadcastFactory implementation instead.

  Removing this code fixes a subtle error condition: in the old local mode
  code, a failure to find the broadcast in the local BlockManager would lead
  to an attempt to deblockify zero blocks, which could lead to confusing
  deserialization or decompression errors when we attempted to decompress
  an empty byte array.  This should never have happened, though: a failure to
  find the block in local mode is evidence of some other error.  The changes
  here will make it easier to debug those errors if they ever happen.

- Add a check that throws an exception when attempting to deblockify an
  empty array.

- Use ScalaCheck to add a test to check that TorrentBroadcast's
  blockifyObject and unBlockifyObject methods are inverses.

- Misc. cleanup and logging improvements.

Author: Josh Rosen <joshrosen@databricks.com>

Closes #2844 from JoshRosen/torrentbroadcast-bugfix and squashes the following commits:

1e8268d [Josh Rosen] Address Reynold's review comments
2a9fdfd [Josh Rosen] Address Reynold's review comments.
c3b08f9 [Josh Rosen] Update TorrentBroadcast tests to reflect removal of special local-mode optimizations.
5c22782 [Josh Rosen] Store broadcast variable's value in the driver.
33fc754 [Josh Rosen] Change blockify/unblockifyObject to accept serializer as argument.
618a872 [Josh Rosen] [SPARK-3958] TorrentBroadcast cleanup / debugging improvements.

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