Use sync.Cond instead of channel for broadcast

[kelindar]

This PR removes the usage of channel for broadcast and instead makes use of sync.Cond primitive directly. Subscribers are also flushed periodically to avoid lock contention. This results in ~2-4x performance improvement depending on the conditions.

Before

cpu: Intel(R) Core(TM) i7-9700K CPU @ 3.60GHz
BenchmarkEvent/1x1-8            21888343               109.0 ns/op         9175415 ev/s        0 B/op          0 allocs/op
BenchmarkEvent/1x10-8            3311071               724.4 ns/op        13803201 ev/s        0 B/op          0 allocs/op
BenchmarkEvent/1x100-8            514850              4750 ns/op          21049985 ev/s        0 B/op          0 allocs/op
BenchmarkEvent/10x1-8            2458360               975.9 ns/op        10245150 ev/s        0 B/op          0 allocs/op
BenchmarkEvent/10x10-8            447559              5236 ns/op          19085895 ev/s        0 B/op          0 allocs/op
BenchmarkEvent/10x100-8            30741             82802 ns/op          12080138 ev/s       14 B/op          0 allocs/op

After

BenchmarkEvent/1x1-8            69634809                38.07 ns/op       26245794 ev/s        0 B/op          0 allocs/op
BenchmarkEvent/1x10-8           15903538               188.2 ns/op        53029988 ev/s       10 B/op          0 allocs/op
BenchmarkEvent/1x100-8           1345704              1679 ns/op          59502067 ev/s       68 B/op          0 allocs/op
BenchmarkEvent/10x1-8            6537975               504.9 ns/op        19796366 ev/s        0 B/op          0 allocs/op
BenchmarkEvent/10x10-8           1740412              1979 ns/op          50472108 ev/s       87 B/op          0 allocs/op
BenchmarkEvent/10x100-8           160111             17376 ns/op          57290578 ev/s     2616 B/op          0 allocs/op