switched implementation to multi-account mode

* instead of starting signal-cli in json-rpc mode with the '-u',
  we start signal-cli without the '-u' parameter (also known as
  multi-account mode). This makes it possible to register a number in
  json-rpc mode.

src/utils/config.go

@@ -6,6 +6,8 @@ import (
 	"io/ioutil"
 )
 
+const MULTI_ACCOUNT_NUMBER string = "<multi-account>"
+
 type JsonRpc2ClientConfigEntry struct {
 	TcpPort      int64  `yaml:"tcp_port"`
 	FifoPathname string `yaml:"fifo_pathname"`