tevent: add tevent_context_wrapper_create() infrastructure

tevent: add tevent_context_wrapper_create() infrastructure

This allows to specify wrapper tevent_contexts, which adds the ability
to run functions before and after the event handler functions.

This can be used to implement impersonation hooks
or advanced debugging/profiling hooks.

We'll undo the 0.9.36 ABI change on the 0.9.37 release
at the end of this patchset.

Signed-off-by: Stefan Metzmacher <metze@samba.org>
Reviewed-by: Ralph Boehme <slow@samba.org>