Add the possibility to execute sling pipes in a background thread

For now the only way to execute a pipe through HTTP is to wait for servlet's response to end.
For very long pipes this is an issue (as well as for the fact all pending changes might impact memory), as well as it is error prone in case the same pipe is executed several times.

this ticket is for adding a new execution mode, that write the running status of the pipe configuration, and makes the pipe executed in a separate thread than the servlet thread.