experimental: Use shared memory pool even when offHeap is not enabled

[andygrove]

Which issue does this PR close?

Closes #996

Rationale for this change

We currently create separate memory pools for each native plan within an executor and we do not take concurrency into account when executors have multiple cores assigned, making this difficult to configure.

What changes are included in this PR?

Always use a shared memory pool per executor.

How are these changes tested?

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 50.00000% with 13 lines in your changes missing coverage. Please review.

Project coverage is 34.20%. Comparing base (c3023c5) to head (34596c5).
Report is 24 commits behind head on main.

Files with missing lines	Patch %	Lines
.../java/org/apache/spark/CometTaskMemoryManager.java	40.90%	10 Missing and 3 partials ⚠️

Additional details and impacted files

@@             Coverage Diff              @@
##               main     #998      +/-   ##
============================================
+ Coverage     34.03%   34.20%   +0.16%     
- Complexity      875      883       +8     
============================================
  Files           112      112              
  Lines         43289    43442     +153     
  Branches       9572     9625      +53     
============================================
+ Hits          14734    14859     +125     
+ Misses        25521    25519       -2     
- Partials       3034     3064      +30     

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