Assigns default memory size for relationship group store

The default size is 10M (or 0.03% size of nodestore if the size is allocated by AutoConfig)
Tagging for version 2.1.6
Back to development version.
2.1.6-SNAPSHOT -> 2.1.7-SNAPSHOT
Merge pull request #3493 from S124568/2.1-add-MMIO-setting-relgroupstore

Assigns default memory size for relationship group store
Merge branch '2.1-maint'