uci: fix session delta isolation

uci: fix session delta isolation

The libuci `uci_set_savedir()` function does not replace, but appends the
requested delta directory to the search path, this causes information leaks
between sessions.

Due to a lack of an official api for this, clear the uci contexts private
delta path before setting the save directory.

Signed-off-by: Jo-Philipp Wich <jo@mein.io>