Fix lifespan and idle time types

Currently, the RemoteCache is accepting lifespan and idle time as uint64_t. Documentation states that setting these values to negative values means unlimited - but uints cannot be negative.

The protocol specifies later that these values should be in seconds, and as unsigned 32-bit integers. However, the server will process them as signed ints. Therefore, some checks should happen on well defined place (throwing exceptions for values that cannot fit into the 31-bit range), and the conversion should be explicit (otherwise, compiler warnings occur).

Fixing this will probably change API.