Reuse the same channel for all precompleted contexts

This avoids making a new channel and immediately closing it. `benchcmp`
says:

    benchmark                      old ns/op     new ns/op     delta
    BenchmarkPrecompleted-4        213           138           -35.21%

    benchmark                      old allocs    new allocs    delta
    BenchmarkPrecompleted-4        3             2             -33.33%

    benchmark                      old bytes     new bytes     delta
    BenchmarkPrecompleted-4        176           80            -54.55%