Drop use of unmodifiable map in arity predictor CTR #2184

Job	Run time
smoke	3m 6s
gremlin-server default	24m 25s
mvn clean install - jdk8	13m 44s
cache gremlin-server docker image	1m 43s
gremlin-server unified	23m 11s
mvn clean install - jdk11	16m 26s
spark core (ubuntu-latest)	12m 8s
spark core (windows-latest)	3m 32s
gremlin-console	5m 40s
neo4j-gremlin	8m 34s
spark gryo (ubuntu-latest)	9m 51s
spark gryo (windows-latest)	4m 49s
gremlin-driver	3m 51s
javascript (ubuntu-latest)	5m 34s
go	6m 2s
python	6m 4s
.NET	6m 11s
	2h 34m 51s

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