WIP: Create separate instance of CometShuffleMemoryAllocator per plan

[andygrove]

Which issue does this PR close?

Closes: #886

Rationale for this change

See issue for details

What changes are included in this PR?

• [x] Create separate memory allocator per shuffle instance
• [x] Update configs and documentation

How are these changes tested?

[parthchandra]

I'm guessing this is the correct issue: #886