[SPARK-3477] Clean up code in Yarn Client / ClientBase

This is part of a broader effort to clean up the Yarn integration code after #2020.

The high-level changes in this PR include:
- Removing duplicate code, especially across the alpha and stable APIs
- Simplify unnecessarily complex method signatures and hierarchies
- Rename unclear variable and method names
- Organize logging output produced when the user runs Spark on Yarn
- Extensively add documentation
- Privatize classes where possible

I have tested the stable API on a Hadoop 2.4 cluster. I tested submitting a jar that references classes in other jars in both client and cluster mode. I also made changes in the alpha API, though I do not have access to an alpha cluster. I have verified that it compiles, but it would be ideal if others can help test it.

For those interested in some examples in detail, please read on.

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***Appendix***

- The loop to `getApplicationReport` from the RM is duplicated in 4 places: in the stable `Client`, alpha `Client`, and twice in `YarnClientSchedulerBackend`. We should not have different loops for client and cluster deploy modes.
- There are many fragmented small helper methods that are only used once and should just be inlined. For instance, `ClientBase#getLocalPath` returns `null` on certain conditions, and its only caller `ClientBase#addFileToClasspath` checks whether the value returned is `null`. We could just have the caller check on that same condition to avoid passing `null`s around.
- In `YarnSparkHadoopUtil#addToEnvironment`, we take in an argument `classpathSeparator` that always has the same value upstream (i.e. `File.pathSeparator`). This argument is now removed from the signature and all callers of this method upstream.
- `ClientBase#copyRemoteFile` is now renamed to `copyFileToRemote`. It was unclear whether we are copying a remote file to our local file system, or copying a locally visible file to a remote file system. Also, even the content of the method has inaccurately named variables. We use `val remoteFs` to signify the file system of the locally visible file and `val fs` to signify the remote, destination file system. These are now renamed `srcFs` and `destFs` respectively.
- We currently log the AM container's environment and resource mappings directly as Scala collections. This is incredibly hard to read and probably too verbose for the average Spark user. In other modes (e.g. standalone), we also don't log the launch commands by default, so the logging level of these information is now set to `DEBUG`.
- None of these classes (`Client`, `ClientBase`, `YarnSparkHadoopUtil` etc.) is intended to be used by a Spark application (the user should go through Spark submit instead). At the very least they should be `private[spark]`.

Author: Andrew Or <andrewor14@gmail.com>

Closes #2350 from andrewor14/yarn-cleanup and squashes the following commits:

39e8c7b [Andrew Or] Address review comments
6619f9b [Andrew Or] Merge branch 'master' of github.com:apache/spark into yarn-cleanup
2ca6d64 [Andrew Or] Improve logging in application monitor
a3b9693 [Andrew Or] Minor changes
7dd6298 [Andrew Or] Simplify ClientBase#monitorApplication
547487c [Andrew Or] Provide default values for null application report entries
a0ad1e9 [Andrew Or] Fix class not found error
1590141 [Andrew Or] Address review comments
45ccdea [Andrew Or] Remove usages of getAMMemory
d8e33b6 [Andrew Or] Merge branch 'master' of github.com:apache/spark into yarn-cleanup
ed0b42d [Andrew Or] Fix alpha compilation error
c0587b4 [Andrew Or] Merge branch 'master' of github.com:apache/spark into yarn-cleanup
6d74888 [Andrew Or] Minor comment changes
6573c1d [Andrew Or] Clean up, simplify and document code for setting classpaths
e4779b6 [Andrew Or] Clean up log messages + variable naming in ClientBase
8766d37 [Andrew Or] Heavily add documentation to Client* classes + various clean-ups
6c94d79 [Andrew Or] Various cleanups in ClientBase and ClientArguments
ef7069a [Andrew Or] Clean up YarnClientSchedulerBackend more
6de9072 [Andrew Or] Guard against potential NPE in debug logging mode
fabe4c4 [Andrew Or] Reuse more code in YarnClientSchedulerBackend
3f941dc [Andrew Or] First cut at simplifying the Client (stable and alpha)