Bluetooth: Mesh: Introduce measures to avoid too frequent flash writes

Both the local sequence number as well as the Replay Protection List
(RPL) are states that may potentially change very often. In order not
to wear out the flash with these updates it makes sense to try to
avoid too frequent writes.

For the local sequence number a simple solution is not to write the
number on every increment. This patch introduces a new Kconfig option
to define after how many increments the sequence number gets written.
When the stack gets initialized it automatically adds the configured
number to the last stored one, thereby guaranteeing that the node
starts off with a number that's larger than the last used one.

The RPL is more problematic, since in principle it needs to be updated
every single time that we receive and process a message. Especially
security sentitive nodes will want this stored immediately to flash.
To give some use-case dependent flexibility, this patch introduces a
new Kconfig option to specify a timeout after which the RPL gets
written to flash.

Signed-off-by: Johan Hedberg <johan.hedberg@intel.com>