proxy,proxy/config: add global TTL feature (#554)

Add a new configuration parameter globalTTL that all models will
inherit. The default value is 0 which matches the currently
functionality to never automatically unload a model.

The model.ttl's default has changed to -1, which means use the global
TTL value. Any model.ttl >=0 is now value with 0 meaning never unload.
This allows a model to override a globalTTL > 0 and be configured to
never unload.

Fixes #459
Closes #512

proxy/proxymanager_test.go

@@ -730,7 +730,7 @@ func TestProxyManager_RunningEndpoint(t *testing.T) {
 		// Verify extended fields are present
 		assert.NotEmpty(t, response.Running[0].Cmd, "cmd should be populated")
 		assert.NotEmpty(t, response.Running[0].Proxy, "proxy should be populated")
-		assert.Equal(t, 0, response.Running[0].TTL, "ttl should default to 0")
+		assert.Equal(t, -1, response.Running[0].TTL, "ttl should default to -1 (use globalTTL)")
 	})
 }