PaymentExitGame contains several redundant plasmaFramework declarations
`PaymentExitGame` inherits from both `PaymentInFlightExitRouter` and `PaymentStandardExitRouter`. All three contracts declare and initialize their own `PlasmaFramework` variable. This pattern can be misleading, and may lead to subtle issues in future versions of the code.\\n`PaymentExitGame` declaration:\\n```\\nPlasmaFramework private plasmaFramework;\\n```\\n\\n`PaymentInFlightExitRouter` declaration:\\n```\\nPlasmaFramework private framework;\\n```\\n\\n`PaymentStandardExitRouter` declaration:\\n```\\nPlasmaFramework private framework;\\n```\\n\\nEach variable is initialized in the corresponding file's constructor.
Introduce an inherited contract common to `PaymentStandardExitRouter` and `PaymentInFlightExitRouter` with the `PlasmaFramework` variable. Make the variable internal so it is visible to inheriting contracts.
```\\nPlasmaFramework private plasmaFramework;\\n```\\n