Bluetooth: Mesh: Add API to manually store pending RPL entries

The current approach with storing RPL by timeout doesn't solve all
issues as the node may loss power before the timer is fired.
In addition to that this may wear out flash quickly if short timeout is
used.

This change adds an API to store the pending RPL entry upon user
request. Additional Kconfig option allows to completely disable timer
so that the whole storing relies on the user.

The mesh stack still stays responsible for outdating RPL entries in case
of IV Index update as this happens implicitly for the user.

Signed-off-by: Pavel Vasilyev <pavel.vasilyev@nordicsemi.no>