Define a Drain::flush method #176

Job	Run time
test (1.61)	41s
test (1.61, nested-values)	40s
test (1.61, std dynamic-keys)	51s
test (1.61, std)	42s
test (stable minus 15 releases)	19s
test (1.61, std nested-values dynamic-keys)	42s
test (1.61, std nested-values dynamic-keys nothreads)	40s
test (stable minus 15 releases, std dynamic-keys)	22s
test (stable minus 15 releases, std nested-values)	23s
test (1.61, std nested-values)	40s
test (1.61, std nothreads)	40s
test (1.61, dynamic-keys)	45s
test (stable minus 15 releases, std nested-values dynamic-keys)	24s
test (stable minus 15 releases, std)	20s
test (stable minus 15 releases, nested-values)	20s
test (stable, nested-values)	22s
test (stable minus 15 releases, std nothreads)	23s
test (stable minus 15 releases, dynamic-keys)	23s
test (stable)	20s
test (stable, dynamic-keys)	22s
test (stable minus 15 releases, std nested-values dynamic-keys nothreads)	20s
test (stable, std nested-values)	24s
test (stable, std)	22s
test (stable, std nested-values dynamic-keys)	22s
test (stable, std nested-values dynamic-keys nothreads)	21s
test (stable, std nothreads)	21s
test (nightly, std)	22s
test (stable, std dynamic-keys)	24s
test (nightly)	20s
test (nightly, dynamic-keys)	23s
test (nightly, std nested-values dynamic-keys)	23s
test (nightly, nested-values)	22s
test (nightly, std dynamic-keys)	21s
test (nightly, std nothreads)	21s
test (nightly, std nested-values dynamic-keys nothreads)	22s
test (nightly, std nested-values)	19s
test (stable, std parking_lot_0_12)	21s
	16m 27s

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