Refactor default lock

[bmwant]

Description

Make cached and cachedmethod functions more concise by introducing default no-op NoLock class thus eliminating duplication in elif lock is None: branches.

Testing

import timeit
setup = """
from cachetools import cached
import time

@cached(cache={})
def dummy_func():
    time.sleep(0.1)
    return 10 ** 10
"""

s = """
dummy_func()
"""

print(timeit.timeit(setup=setup, stmt=s, number=100_000))
# 0.14263679101713933
# 0.15603854099754244

There is no significant overhead with running no-op locking/unlocking (only 1/100th of a second) and taking into account that caching is always applied to already "heavy" functions it's negligible.

[bmwant]

@tkem let me know what do you think about this change, in my opinion it improves readability and makes it easier to maintain/update in the future.