mcumgr: Add latency control functionality

This change adds a posibility to enable low latency connection
parameters for BT when SMP commands are handled.

Support for this functionality is disabled by the default and
can be enabled by CONFIG_MCUMGR_SMP_BT_LATENCY_CONTROL=y option.

Signed-off-by: Emil Obalski <emil.obalski@nordicsemi.no>