Core: release resources on exit, the saga continue

this patch makes to release many resources from the core subsystems
such as epoll_states, vhost fdt, clock, buffers cache, etc.

This is still work in process and the pending areas are:

 - Global scheduler
 - Plugins -> per worker exit
 - Plugin Events
 - Plugins shared libraries

Signed-off-by: Eduardo Silva <eduardo@monkey.io>