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.
Merge pull request #514 from candrews/patch-2

GROOVY-7062: Use low concurrencyLevel in rarely updated maps