#397: Replace singletons with @functools.lru_cache() decorator

• Replace BlockchainParams() singleton with @functools.lru_cache()
• Replace HeadBlock() singleton with @functools.lru_cache()
• Replace UDValue() singleton with @functools.lru_cache()
• Replace CurrencySymbol() singleton with @functools.lru_cache()
• Replace ClientInstance() singleton with @functools.lru_cache()

Close #397 (closed).