Revert "Adds the concept of constraint indexes."

This reverts commit 1fb4e714799f24983cbd2ca2e287bf3981541abd.
This reverts commit 453d4cb88220e9d5c74262a7d0ba6c586e9e0a32.
Adds REST api for constraint handling

o REST API for GET, POST, DELETE of constraints
o Adds ConstraintType enum to ConstraintDefinition
o Change: Creating the same constraint twice is a ConstraintViolation
o Adds StmCtx.getConstraints()
o Tests proper TxState handling of the various getConstrains() methods