RPC calls optimization

[stepansnigirev]

We do way too many calls to the RPC.
At the moment when I start Specter I see that rpc is called 300+ times - it will be a nightmare for a remote node.
I have plenty of wallets and transactions, but still this is something we should optimize.

I started doing it in this PR, reduced number of calls to ~30 per request (roughly 2 per wallet). And more if the wallet is selected.

Changes:

• don't check transactions that we already checked - use listsinceblock instead of listtransactions
• -1 call to getbalances
• don't create wallets if they are already there
• wallet.update() method to check balances and addresses
• wallet.get_info() method to get rescan process
• keep full reload of the wallet manager if user or chain have changed

Also added an RPC counter per request that we can uncomment during the development and see which and how many calls are done.

[ben-kaufman]

Looks good :)