Reduce Rpc Continuation Queue Locking

[YulerB]

Proposed Changes

• Updated RpcContinuationQueue.cs.
• Removed locking and replaced with an atomic operation.
• This reduces memory and mean time to execute.
• Also making the variable that is being locked private.
• Also removes the chance of a null exception in code, where the output of Next() is not checked.
• Added tests.
• Benchmark test code and outcome added to comments below.

Types of Changes

What types of changes does your code introduce to this project?

• Bugfix (non-breaking change which fixes issue #NNNN)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Documentation (correction or otherwise)
• Cosmetics (whitespace, appearance)
• Performance Update

Checklist

• I have read the CONTRIBUTING.md document
• I have signed the CA (see https://cla.pivotal.io/sign/rabbitmq)
• All tests pass locally with my changes
• I have added tests that prove my fix is effective or that my feature works
• I have added necessary documentation (if appropriate)
• Any dependent changes have been merged and published in related repositories

Further Comments

BenchmarkDotNet=v0.10.14, OS=Windows 10.0.17134
Intel Core i5-6200U CPU 2.30GHz (Skylake), 1 CPU, 4 logical and 2 physical cores
Frequency=2343750 Hz, Resolution=426.6667 ns, Timer=TSC
  [Host]     : .NET Framework 4.6.1 (CLR 4.0.30319.42000), 32bit LegacyJIT-v4.7.3132.0  [AttachedDebugger]
  Job-JWVBYN : .NET Framework 4.6.1 (CLR 4.0.30319.42000), 32bit LegacyJIT-v4.7.3132.0
Runtime=Clr  IsBaseline=True  
Method     Mean        Error           StdDev       Scaled      Rank       Gen 0        Allocated
RpcNew    26.94 ns    0.3565 ns    0.3334 ns   1.00          1             0.0076      12 B
RpcOld      73.05 ns    2.7595 ns    3.1779 ns   1.00          1             0.0178      28 B

Bench mark code:
bench.cs.txt

[kjnilsson]

This looks good to me but as usual with these type of changes it would be good to get a couple of eyes on it.

[lukebakken]

Please ensure that the code formatting follows the rest of the project's style by placing the opening curly on a new line.

[lukebakken]

Please remove extra blank lines.

[michaelklishin]

Thank you @YulerB.