Limit parallel script processing thread pool to 4 by default

[prasannavl]

/kind chore

• Limits parallel execution threads of Bitcoin threads to 4 by default.
• We do not utilise this for DeFi TXs (unfortunately), and usually is just a wasted threads sitting around unless there are many many pure UTXO TXs. 4 threads should be sufficient for most practical purposes.
• Doing this cleanup as a precursor to add a common worker pool for most other thread related activites

[Bushstar]

Perhaps it is better to set DEFAULT_SCRIPTCHECK_THREADS to 4?

Then users can still override and set to 0 to let the client use GetNumCores(). This change will always cap the number of cores to 4 regardless of user settings.

[prasannavl]

Good catch. Switching to use DEFAULT_SCRIPTCHECK_THREADS

[prasannavl]

Overlooked earlier - users can already override. This only changes the behaviour of the auto (0) config. Rest follow the same. Cannot change script default, since that's the one that will have a behaviour change across different hardware and also requires turning it into a non-const.

[prasannavl]

Ignores the additional reverted commits - initial is the most optimal version, I believe.