### What changes were proposed in this pull request?

In the current implementation of `SparkShellLoggingFilter`, if the log level of the root logger and the log level of a message are different, whether a message should logged is decided based on log4j's configuration but whether the message should be output to the REPL's console is not cared.
So, if the log level of the root logger is `DEBUG`, the log level of REPL's logger is `WARN` and the log level of a message is `INFO`, the message will output to the REPL's console even though `INFO < WARN`.
https://github.com/apache/spark/pull/26798/files#diff-bfd5810d8aa78ad90150e806d830bb78L237

The ideal behavior should be like as follows and implemented them in this change.

1. If the log level of a message is greater than or equal to the log level of the root logger, the message should be logged but whether the message is output to the REPL's console should be decided based on whether the log level of the message is greater than or equal to the log level of the REPL's logger.

2. If a log level or custom appenders are explicitly defined for a category, whether a log message via the logger corresponding to the category is logged and output to the REPL's console should be decided baed on the log level of the category.
We can confirm whether a log level or appenders are explicitly set to a logger for a category by `Logger#getLevel` and `Logger#getAllAppenders.hasMoreElements`.

### Why are the changes needed?

This is a bug breaking a compatibility.

#9816 enabled REPL's log4j configuration to override root logger but #23675 seems to have broken the feature.
You can see one example when you modifies the default log4j configuration like as follows.
```
# Change the log level for rootCategory to DEBUG
log4j.rootCategory=DEBUG, console

...
# The log level for repl.Main remains WARN
log4j.logger.org.apache.spark.repl.Main=WARN
```
If you launch REPL with the configuration, INFO level logs appear even though the log level for REPL is WARN.
```
・・・

19/12/08 23:31:38 INFO Utils: Successfully started service 'sparkDriver' on port 33083.
19/12/08 23:31:38 INFO SparkEnv: Registering MapOutputTracker
19/12/08 23:31:38 INFO SparkEnv: Registering BlockManagerMaster
19/12/08 23:31:38 INFO BlockManagerMasterEndpoint: Using org.apache.spark.storage.DefaultTopologyMapper for getting topology information
19/12/08 23:31:38 INFO BlockManagerMasterEndpoint: BlockManagerMasterEndpoint up
19/12/08 23:31:38 INFO SparkEnv: Registering BlockManagerMasterHeartbeat

・・・
```
Before #23675 was applied, those INFO level logs are not shown with the same log4j.properties.

### Does this PR introduce any user-facing change?

Yes. The logging behavior for REPL is fixed.

### How was this patch tested?

Manual test and newly added unit test.

Closes #26798 from sarutak/fix-spark-shell-loglevel.

Authored-by: Kousuke Saruta <sarutak@oss.nttdata.com>
Signed-off-by: Marcelo Vanzin <vanzin@cloudera.com>