Bluetooth: Mesh: Update defaults - proxy filter, msg cache

This updates the default proxy filter size to 16. Previous value of 3
is too less for the most practical uses and demos. The default proxy
filter type is accept list type and in this mode proxy server rejects
incoming messages from source addresses not in the accept list. The
addresses are added to the accept list when proxy client sends
messages to unicast addresses or manually adds certain addresses to
the accept list. Once this list is full more addresses cannot be added.

This also updates the default network message cache size to 32. The
network message cache helps in preventing duplicate messages getting
repeatedly relayed and helps in reducing unnecessary network traffic.
Previous value of 10 is quite less for most usecases and makes the
node appear to generate much more traffic in mixed network. The
updated value should suffice for most use cases.

Additional explannation is added in Kconfig to help users understand
the significance of this setting.

These two changes result in 176 bytes of additional RAM usage in mesh
samples.

Signed-off-by: Omkar Kulkarni <omkar.kulkarni@nordicsemi.no>