Manager fixes (#126)

* Fix uninitialized mutex & cvar
* Logging + test fix

Author: bretambrose

include/aws/http/http.h

@@ -44,6 +44,7 @@ enum aws_http_errors {
     AWS_ERROR_HTTP_CONNECTION_MANAGER_VENDED_CONNECTION_UNDERFLOW,
     AWS_ERROR_HTTP_SERVER_CLOSED,
     AWS_ERROR_HTTP_PROXY_TLS_CONNECT_FAILED,
+    AWS_ERROR_HTTP_CONNECTION_MANAGER_SHUTTING_DOWN,
 
     AWS_ERROR_HTTP_END_RANGE = 0x0C00,
 };