fix(live update): pages and REST API last update check (#957)

* use in memory cache to avoid reading filesystem at each call
* do not read last update date in database more than once every 3 second (can be configured)
* synchronize on page dedicated object instead of class object
* remove the possibility for the groovy classloader of a page to be created twice

Covers [RUNTIME-1408](https://bonitasoft.atlassian.net/browse/RUNTIME-1408

)

Co-authored-by: Emmanuel Duchastenier <emmanuel.duchastenier@bonitasoft.com>