Improve Spill Performance: mmap the spill files

[zebsme]

Which issue does this PR close?

Rationale for this change

What changes are included in this PR?

Are these changes tested?

Are there any user-facing changes?

[zebsme]

This draft can reproduce the following errors:

FAIL [   0.054s] datafusion::core_integration memory_limit::test_disk_spill_limit_not_reached
FAIL [  21.391s] datafusion::core_integration memory_limit::test_in_mem_buffer_almost_full
FAIL [  48.291s] datafusion::core_integration memory_limit::test_stringview_external_sort
FAIL [   0.334s] datafusion::fuzz fuzz_cases::aggregate_fuzz::test_single_mode_aggregate_with_spill
FAIL [   0.018s] datafusion-physical-plan aggregates::tests::test_aggregate_with_spill_if_necessary
FAIL [   0.090s] datafusion-physical-plan sorts::sort::tests::test_sort_spill_utf8_strings 

These failures share similar output:

──── STDERR:             datafusion::core_integration memory_limit::test_in_mem_buffer_almost_full

thread 'memory_limit::test_in_mem_buffer_almost_full' panicked at datafusion/core/tests/memory_limit/mod.rs:496:32:
called `Result::unwrap()` on an `Err` value: ResourcesExhausted("Failed to allocate additional 3806586 bytes for ExternalSorterMerge[0] with 0 bytes already allocated for this reservation - 2872588 bytes remain available for the total pool")

[zebsme]

Apply these code changes to make all tests pass:

datafusion/datafusion/physical-plan/src/joins/sort_merge_join.rs

Lines 2310 to 2320 in 5bac728

	// >>> REMOVE THIS BLOCK <<<
	// -------------------------------------------
	let file = File::open(spill_file.path())?;
	let mmap = unsafe { Mmap::map(&file)? };
	let bytes = bytes::Bytes::from_owner(mmap);
	let buffer = Buffer::from(bytes);
	let decoder = IPCBufferDecoder::new(buffer);
	// -------------------------------------------
	// >>> UNCOMMENT BELOW <<<
	// let decoder = IPCBufferDecoder::new(spill_file.path());

datafusion/datafusion/physical-plan/src/spill/mod.rs

Lines 40 to 41 in 5bac728

	// >>> REMOVE THIS BLOCK <<<
	// -------------------------------------------------

datafusion/datafusion/physical-plan/src/spill/mod.rs

Lines 128 to 129 in 5bac728

	// // >>> UNCOMMENT BELOW <<<
	// // -------------------------------------------------------------