Remove CBaseChainParams::NetworkID()

~~Continues #4802 and~~ Includes and closes #4803 
Completes the set of PRs #3823 #3824 #4333 #4801 #4802 #4803 
Small readability nit: Instead of having an array here at all, lets

    static boost::filesystem::path pathCached;
    static boost::filesystem::path pathCachedNetSpecific;

then below

    fs::path &path = fNetSpecific ? pathCachedNetSpecific : pathCached;

and

    pathCached = boost::filesystem::path();
    pathCachedNetSpecific = boost::filesystem::path();