Endpoint improvements

This MR refactors the si-audio-endpoint module to use internally a si-audio-adapter, and so avoids code duplication when setting audio format in both modules.

In addition to this, both policy-endpoint-device.lua and policy-endpoint-client.lua scripts have been update to look similar to policy-node.lua. The fix to delay the rescan when an endpoint link cannot be removed (because it is still being activated) is also included.