Add a masternode return of investment RPC

[techy2]

update PR #2929 to current "master" branch
accurate CLI function for staking and masternode ROI
getroi
{
"3 hour avg ROI": "23.9%",
"30 min stk ROI": "29.5%",
"network stake": "8,564,072",
"--------------": "--------------",
"masternode ROI": "16.0%",
"tot collateral": "19,150,000",
"enabled nodes": "1915",
"blocks per day": "1401.1"
}

[Duddino]

Thanks for the PR. The code needs to be formatted and redundant comments cleaned up.
I'm not sure if this RPC is actually needed, It seems like something a third party tool like toolbox.pivx.org would provide, rather than the core wallet.

[Duddino]

Suggested change

	// returns count enabled or zero on error
	// returns count enabled or zero on error

This says it will return zero on error, but later it returns negative numbers on error

[techy2]

see line 76

[Fuzzbawls]

I think that this kind of functionality is better suited to a website and not the core wallet. this introduces features that are not reasonably tested in a meaningful way (code coverage / regression tests).

[techy2]

I don't have any PIVX so it make no difference to me. I provided this as a courtesy.
This is what it looks like fully implemented if you bring your code up to snuff.