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

.gitignore

@@ -2,6 +2,7 @@
 .DS_Store
 .grin*
 node*
+!node_clients
 target
 Cargo.lock
 *.iml