Remove `id.Append()`.

Benchmarks: 1510 ns/op for `GenerateBytes()`, compared to 1425 ns/op
for `Append()`. `newHekaMessage()` with `GenerateBytes()` took
3765 ns/op and 16 allocs/op, compared to 3515 ns/op and 15 allocs/op
for `Append()`.
Clean up log emitter initialization.

Clarify `TryClose` behavior.