Tiner's overhead costing

[gusye1234]

Now, tiner is using the context mechanism tricky in Python, which seems have some overheads.
The reporting time form tiner is not the exact running time of the block, roughly 1e-3s more

tiner/tests/test_tiner.py

Lines 9 to 13 in f0312c5

	with tiner("test:block"):
	s = perf_counter()
	sleep(duration)
	d = perf_counter() - s
	assert (tiner.get("test:block") - d) < 1e-3