Support Pallet Instances in Relay (#238)

* Sketch out how to support different bridge pallet instances

* Create a common interface for using pallet instances

* Start introducing generic instance parameter

Attemps to make the compiler happy, but I'm having second thoughts about
this approach. Commiting now as a way to have a checkpoint, but I think
I'm going to need to re-consider my approach here.

Ideally I want a change which introduces minimal changes, but this seems
to be propagating around the codebase in ways I don't want.

* Use trait objects instead of generics

* Implement traits for Boxed trait objects

This is done in order to statisfy trait bounds by types
which use these new trait objects

* Remove Clone usage for sync parameters

* Remove implementation of Default for sync params

* Require that BridgeInstance implements Debug

* Ensure that BridgeInstance trait implements Send/Sync

* Add documentation related to instances

* Rust Fmt

* Remove needless format

* Make instance CLI option case insensitive

* Replace `with_*` constructors with `new`

* Clean up usage of instance structs

* Enforce a default instance in the CLI params

* Build sync params as we process input from CLI

* Remove case insensitivity from sub-tx-mode

I think this should happen, but maybe as part of a different PR

* Process default Eth contract deployment config in CLI

* Build EthereumExchangeParams in CLI

* Process EthereumExchangeSubmitParams params in CLI

bridges/relays/ethereum/src/cli.yml

@@ -43,6 +43,17 @@ subcommands:
                 long: sub-signer-password
                 value_name: SUB_SIGNER_PASSWORD
                 help: The password for the SURI of secret key to use when transactions are submitted to the Substrate node.
+            - sub-pallet-instance: &sub-pallet-instance
+                long: instance
+                short: i
+                value_name: PALLET_INSTANCE
+                help: The instance of the bridge pallet the relay should follow.
+                takes_value: true
+                case_insensitive: true
+                possible_values:
+                  - Rialto
+                  - Kovan
+                default_value: Rialto
             - no-prometheus: &no-prometheus
                 long: no-prometheus
                 help: Do not expose a Prometheus metric endpoint.