Implement z_gettotalsaplingnotescount rpc call

[kowalski]

We would like to ask to implement a rpc call that will return a length of wtx.mapSaplingNoteData structure. We would like to have a very "cheap" way of knowing if znode received a note since last time we check.

Today we (Zbay team) use z_getreceivedbyaddress call for each observed address which we call frequently. This proves to be very expensive in terms of CPU usage, because every call requires decrypting all the notes in the wallet.

If there is an endpoint giving the overall count we can just call it first and immediately know that there is no need for scanning each address (if count is the same as last time we've checked).
We will only rescan addresses (using z_getreceivedbyaddress) if the count has changed since the last call.

This will allow us to drastically reduce number of calls Zbay is making and consequently reduce the footprint it has for the OS.