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 merge request !182 (merged)

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mentioned in commit 0b39ca8d

mentioned in commit 2e0036d4

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mentioned in merge request !211 (merged)

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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

changed the description

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

changed title from Replace singleton with @functools.lru_cache() decorator to Replace singletons with @functools.lru_cache() decorator

marked the checklist item ClientInstance as completed

mentioned in commit 32cf9988

mentioned in commit 6b1dc44f

mentioned in commit 68de8cc5

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closed with merge request !211 (merged)