Bluetooth: Mesh: Update mesh shell's config

This updates some of the defaults in the mesh shell's config to allow
various Mesh 1.1 models to be used in the shell.

EPA provisioning and DFU requires much larger multisegment messages,
therefore support for maximum sized message is enabled by default and
stack sizes are increased working values.

Additionally, log buffer is increased and thread sleep time is reduced
to avoid dropping of useful logs.

Signed-off-by: Omkar Kulkarni <omkar.kulkarni@nordicsemi.no>
Signed-off-by: Pavel Vasilyev <pavel.vasilyev@nordicsemi.no>