Replace singletons with @functools.lru_cache() decorator

• CurrencySymbol
• BlockchainParams
• HeadBlock
• UDValue
• ClientInstance

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Use @functools.lru_cache(maxsize=1) decorator:

• Blog article: Simple singletons
• Python doc: functools.lru_cache

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mentioned in commit 0b39ca8d

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changed title from Get rid of singleton usage to Get rid of singleton usage, replace with @functools.lru_cache decorator

changed title from Get rid of singleton usage, replace with @functools.lru_cache decorator to Get rid of singleton usage, replace with @functools.lru_cache() decorator

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