Optimise for high load

[haf]

Goal: Support shipping 10k messages per second to stackdriver.

Stage 1:

• Remove closure allocation in logWithTimeout (by synchronising immediately on buffer)
  • Should remove one Alt.choose, one timeout item Alt

Stage 2:

• Fix Pipe.scan is not single-threaded... #310 and see what optimisations are available

[haf]

BenchmarkDotNet=v0.10.14, OS=Windows 10.0.14393.2068 (1607/AnniversaryUpdate/Redstone1)
Intel Core i7-7920HQ CPU 3.10GHz (Kaby Lake), 1 CPU, 8 logical and 4 physical cores
Frequency=3023437 Hz, Resolution=330.7494 ns, Timer=TSC
.NET Core SDK=2.1.100
  [Host]     : .NET Core 2.0.5 (CoreCLR 4.6.26020.03, CoreFX 4.6.26018.01), 64bit RyuJIT DEBUG
  Job-TVKGOY : .NET Framework 4.7.1 (CLR 4.0.30319.42000), 64bit RyuJIT-v4.7.2563.0
  Job-LLLTYO : .NET Core 2.0.5 (CoreCLR 4.6.26020.03, CoreFX 4.6.26018.01), 64bit RyuJIT

Concurrent=True  Server=True  InvocationCount=128  
LaunchCount=1  WarmupCount=1024  

                Method | Runtime |     Mean |     Error |     StdDev |   Median | Rank |
---------------------- |-------- |---------:|----------:|-----------:|---------:|-----:|
 LogHelloWorld_WithAck |     Clr | 250.6 ns |  5.053 ns |   7.084 ns | 247.7 ns |    2 |
  LogHelloWorld_WithBP |     Clr | 156.1 ns |  3.137 ns |   4.883 ns | 155.8 ns |    1 |
  LogHelloWorld_Simple |     Clr | 293.7 ns |  6.522 ns |  10.715 ns | 289.9 ns |    3 |
 LogHelloWorld_WithAck |    Core | 519.0 ns | 90.334 ns | 266.351 ns | 433.2 ns |    4 |
  LogHelloWorld_WithBP |    Core | 319.0 ns | 46.788 ns | 137.954 ns | 280.9 ns |    3 |
  LogHelloWorld_Simple |    Core | 459.6 ns | 63.946 ns | 173.968 ns | 374.0 ns |    4 |