Add Kconfig option to randomize the initial ACK timeout, as specified in
RFC 7252. The option is enabled by default.
Additionally, finetune the default value of COAP_INIT_ACK_TIMEOUT_MS
option, to match the default ACK_TIMEOUT value specified by the RFC
7252. The RFC does not specify the minimum/maximum value of the
ACK_TIMEOUT parameter, but only suggests it should be no lower than 1
second, so adjust the option range to reflect this.
Signed-off-by: Robert Lubos <robert.lubos@nordicsemi.no>