[SPARK-4138][SPARK-4139] Improve dynamic allocation settings

This should be merged after #2746 (SPARK-3795).

**SPARK-4138**. If the user sets both the number of executors and `spark.dynamicAllocation.enabled`, we should throw an exception.

**SPARK-4139**. If the user sets `spark.dynamicAllocation.enabled`, we should use the max number of executors as the starting number of executors because the first job is likely to run immediately after application startup. If the latter is not set, throw an exception.

Author: Andrew Or <andrew@databricks.com>

Closes #3002 from andrewor14/yarn-set-executors and squashes the following commits:

c528fce [Andrew Or] Merge branch 'master' of github.com:apache/spark into yarn-set-executors
55d4699 [Andrew Or] Bug fix: `isDynamicAllocationEnabled` was always false
2b0ccec [Andrew Or] Start the number of executors at the max
022bfde [Andrew Or] Guard against incompatible settings of number of executors
[Minor] A few typos in comments and log messages

Author: Andrew Or <andrewor14@gmail.com>
Author: Andrew Or <andrew@databricks.com>

Closes #3021 from andrewor14/typos and squashes the following commits:

daaf417 [Andrew Or] Merge branch 'master' of github.com:apache/spark into typos
4838ae4 [Andrew Or] Merge branch 'master' of github.com:apache/spark into typos
026d426 [Andrew Or] Merge branch 'master' of github.com:andrewor14/spark into typos
a81ae8f [Andrew Or] Some typos
[SPARK-4155] Consolidate usages of <driver>

We use "\<driver\>" everywhere. Let's not do that.

Author: Andrew Or <andrew@databricks.com>

Closes #3020 from andrewor14/consolidate-driver and squashes the following commits:

c1c2204 [Andrew Or] Just use "<driver>" for local executor ID
3d751e9 [Andrew Or] Consolidate usages of <driver>
Minor style hot fix after #2711

I had planned to fix this when I merged it but I forgot to. witgo

Author: Andrew Or <andrew@databricks.com>

Closes #3018 from andrewor14/command-utils-style and squashes the following commits:

c2959fb [Andrew Or] Style hot fix
[SPARK-4153][WebUI] Update the sort keys for HistoryPage

Sort "Started", "Completed", "Duration" and "Last Updated" by time.

Author: zsxwing <zsxwing@gmail.com>

Closes #3014 from zsxwing/SPARK-4153 and squashes the following commits:

ec8b9ad [zsxwing] Sort "Started", "Completed", "Duration" and "Last Updated" by time