Make default subscription streaming option as Parallel

Hi,

By default, currently streaming of in-progress transactions for
subscriptions is disabled. All transactions are fully decoded on the
publisher before being sent to the subscriber. This approach can lead
to increased latency and reduced performance, particularly under heavy
load. By default, we could enable the parallel streaming option for
subscriptions. By doing this, incoming changes will be directly
applied by one of the available parallel apply workers. This method
significantly improves the performance of commit operations.

I conducted a series of tests using logical replication, comparing SQL
execution times with streaming set to both parallel and off. The tests
varied the logical_decoding_work_mem setting and included the
following scenarios: a) Insert, b) Delete, c) Update, d) rollback 5%
records, e) rollback 10% records, f) rollback 20% records, g) rollback
50% records. I have written tap tests for the same, the attached files
can be copied to src/test/subscription/t and the
logical_decoding_work_mem configuration and streaming option in create
subscription command should be changed accordingly before running the
tests. The tests were executed 5 times and the average of them was
taken.
The execution time is in seconds.

Insert 5kk records
Logical Decoding mem | Parallel | off | % Improvement
-------------------------------|-------------|---------------|------------------------
64 KB | 37.304 | 69.465 | 46.298
256 KB | 36.327 | 70.671 | 48.597
64 MB | 41.173 | 69.228 | 40.526

Delete 5kk records
Logical Decoding mem | Parallel | off | % Improvement
-------------------------------|-------------|---------------|------------------------
64 KB | 42.322 | 69.404 | 39.021
256 KB | 43.250 | 66.973 | 35.422
64 MB | 44.183 | 67.873 | 34.903

Update 5kk records
Logical Decoding mem | Parallel | off | % Improvement
-------------------------------|-------------|---------------|------------------------
64 KB | 93.953 | 127.691 | 26.422
256 KB | 94.166 | 128.541 | 26.743
64 MB | 93.367 | 134.275 | 30.465

Rollback 05% records
Logical Decoding mem | Parallel | off | % Improvement
-------------------------------|-------------|---------------|------------------------
64 KB | 36.968 | 67.161 | 44.957
256 KB | 38.059 | 68.021 | 44.047
64 MB | 39.431 | 66.878 | 41.041

Rollback 10% records
Logical Decoding mem | Parallel | off | % Improvement
-------------------------------|-------------|---------------|------------------------
64 KB | 35.966 | 63.968 | 43.775
256 KB | 36.597 | 64.836 | 43.554
64 MB | 39.069 | 64.357 | 39.292

Rollback 20% records
Logical Decoding mem | Parallel | off | % Improvement
-------------------------------|-------------|---------------|------------------------
64 KB | 37.616 | 58.903 | 36.139
256 KB | 37.330 | 58.606 | 36.303
64 MB | 38.720 | 60.236 | 35.720

Rollback 50% records
Logical Decoding mem | Parallel | off | % Improvement
-------------------------------|-------------|---------------|------------------------
64 KB | 38.999 | 44.776 | 12.902
256 KB | 36.567 | 44.530 | 17.882
64 MB | 38.592 | 45.346 | 14.893

The machine configuration that was used is also attached.

The tests demonstrate a significant performance improvement when using
the parallel streaming option, insert shows 40-48 %improvement, delete
shows 34-39 %improvement, update shows 26-30 %improvement. In the case
of rollback the improvement is between 12-44%, the improvement
slightly reduces with larger amounts of data being rolled back in this
case. If there's a significant amount of data to roll back, the
performance of streaming in parallel may be comparable to or slightly
lower in some instances. However, this is acceptable since commit
operations are generally more frequent than rollback operations.

One key point to consider is that the lock on transaction objects will
be held for a longer duration when using streaming in parallel. This
occurs because the parallel apply worker initiates the transaction as
soon as streaming begins, maintaining the lock until the transaction
is fully completed. As a result, for long-running transactions, this
extended lock can hinder concurrent access that requires a lock.

Since there is a significant percentage improvement, we should make
the default subscription streaming option parallel. Attached patch has
the change for the same.
Thoughts?

All of these tests were conducted with both the publisher and
subscriber on the same host. I will perform additional tests with one
of the logical replication nodes on a different host and share the
results later.

Regards,
Vignesh