experimental wallet support above 18 million

[Tay8NWWFKpz9JT4NXU0w]

DO NOT MERGE TO MASTER OR INCLUDE IN ANY RELEASE.
THIS PR IS MEANT FOR TESTING ONLY
IT DISABLES SOME VALIDATION CHECKS AND ALSO MODIFIES MINING REWARDS

Scope of changes:

• Mining reward set to 15 million, to speed up wallet overflow
• Wallet support for processing of incoming transfers which in total exceed 18 million
• Support for creating transfers from such wallets, which are currently capped at 9 million
• Changes to RPC calls to allow display of balances above 18 million. They are now returned as string.