Provide wallet 'plugin' architecture (#1983)

* remove receive_coinbase fn from wallet to wallet client

* start moving clients into separate mod

* rustfmt

* move wallet client + start listener into wallet 'clients'

* rustfmt

* refactor API to make it more modular and completely decouple sending

* rustfmt

* further decouple API from sending methods

* rustfmt

* remove wallet to wallet client trait

* rustfmt

* rename / refactor client + adapters

* rustfmt

* add adapter concept

* add midding node_clients dir

* add file and null adapters

* rustfmt

* remove receive from owner api

* factor out receiving slates into trait

* rustfmt

* adding listen trait

* rustfmt

* change listener to use trait

* rustfmt

* add test for file-based exchange

* replace http api send command

* rustfmt

* move controller out of libwallet and into top-level wallet dir

* rustfmt

* add moved controller

doc/wallet/design/design.md

@@ -24,7 +24,7 @@ the code is organized into the following components (from highest-level to lowes
 * **libTx** - Library that provides lower-level transaction building, rangeproof and signing functions, highly-reusable by wallet implementors.
 * **Wallet Traits** - A set of generic traits defined within libWallet and the `keychain` crate . A wallet implementation such as Grin's current
   default only needs to implement these traits in order to provide a wallet:
-  * **WalletToNodeClient** - Defines communication between the wallet, a running grin node and/or other wallets
+  * **NodeClient** - Defines communication between the wallet, a running grin node and/or other wallets
   * **WalletBackend** - Defines the storage implementation of the wallet
   * **KeyChain** - Defines key derivation operations
   
@@ -66,22 +66,22 @@ pub fn retrieve_outputs<T: ?Sized, C, K>(
 ) -> Result<Vec<OutputData>, Error>
 where
 !·T: WalletBackend<C, K>,
-!·C: WalletToNodeClient,
+!·C: NodeClient,
 !·K: Keychain,
 {  
 ```
 
 With `T` in this instance being a class that implements  the `WalletBackend` trait, which is further parameterized with implementations of
-`WalletToNodeClient` and `Keychain`.
+`NodeClient` and `Keychain`.
 
 There is currently only a single implementation of the Keychain trait within the Grin code, in the `keychain` crate exported as `ExtKeyChain`.
 The `Keychain` trait makes several assumptions about the underlying implementation, particularly that it will adhere to a
 [BIP-38 style](https://github.com/bitcoin/bips/blob/master/bip-0038.mediawiki) 'master key -> child key' model.
 
-There are two implementations of `WalletToNodeClient` within the code, the main version being the `HTTPWalletToNodeClient` found within `wallet/src/client.rs` and
-the seconds a test client that communicates with an in-process instance of a chain. The WalletToNodeClient isolates all network calls, so upgrading wallet
+There are two implementations of `NodeClient` within the code, the main version being the `HTTPNodeClient` found within `wallet/src/client.rs` and
+the seconds a test client that communicates with an in-process instance of a chain. The NodeClient isolates all network calls, so upgrading wallet
 communication from the current simple http interaction to a more secure protocol (or allowing for many options) should be a simple
-matter of dropping in different `WalletToNodeClient` implementations.
+matter of dropping in different `NodeClient` implementations.
 
 There are also two implementations of `WalletBackend` within the code at the base of the `wallet` crate. `LMDBBackend` found within
 `wallet/src/lmdb_wallet.rs` is the main implementation, and is now used by all grin wallet commands. The earlier `FileWallet` still exists

doc/wallet/design/wallet-arch.puml

@@ -40,7 +40,7 @@ folder "Provided by Grin" as services {
 package "Traits Implemented by Wallets" as traits {
 	database "WalletBackend" as wallet_backend
 	database "KeyChain" as keychain
-	component "WalletToNodeClient" as wallet_client
+	component "NodeClient" as wallet_client
 }
 
 note left of wallet_client