Add GraphQL support

[rimutaka]

GraphQL is needed for the cloud part of STM.

• add custom scalar support for u64 and other scalar support
• test it works with the existing structs in S3 (HashMap support)
• hide graphql behind a feature so that it is not included in the standalone client
• replace fork with the base in Cargo.toml (https://github.com/graphql-rust/juniper.git / https://github.com/rimutaka/juniper.git)

[rimutaka]

Depends on resolution of issue graphql-rust/juniper#1071 and PR graphql-rust/juniper#1070. If that is rejected I'll have to make custom impl for HashSet.