net: coap: Randomize initial ACK timeout

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>