s3: vfs: add smb_vfs_ev_glue

s3: vfs: add smb_vfs_ev_glue

This adds VFS helper functions and that work on a struct smb_vfs_ev_glue
object which bundles two event contexts and a few threadpools.

This will be used to streamline the use of impersonating wrappers
in the SMB_VFS.

Notice the verbose comments in source3/smbd/vfs.c.

This will allow us to introduce path based async operations
to the SMB_VFS layer.

Pair-Programmed-With: Stefan Metzmacher <metze@samba.org>

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