Use low concurrencyLevel in rarely updated maps

The maps in ExpandoMetaClass are very rarely updated, and when they are, it's usually not by multiple concurrent threads. Therefore, using a lower concurrency level would result in less memory being used with no performance penalty. Since there are so many ExpandoMetaClass instances at runtime, this per instance savings is significant for a whole application.
[artifactory-release] Next development version
Merge pull request #514 from candrews/patch-2

GROOVY-7062: Use low concurrencyLevel in rarely updated maps
Use a new class loader for each GStringTemplateScript class

See GROOVY-7017
Merge pull request #501 from candrews/GStringTemplateEngineClassLoader

GROOVY-7017: Use a new class loader for each GStringTemplateScript class
GROOVY-7041: Fix spread operator leaving an operand on stack with @CompileStatic