added actor messaging pressure benchmark

[Aaronontheweb]

Changes

purpose of this benchmark is not to measure processing time - it's meant to measure allocation pressure created by Akka.NET infrastructure:

• Envelopes
• Mailbox and MessageDispatcher overhead
• ActorCell and ActorBase invocation overhead

Ideally the memory pressure, given a const string as messaging input, should be close to zero - but between mailbox queue segment allocations, delegate + closure allocations, and probably some boxing we know that's not the case. Purpose of this benchmark is to measure that impact across threads.

N.B. - this benchmark makes no attempt to measure processing time nor does it wait for actors to process all messages. That is by design.

Checklist

For significant changes, please ensure that the following have been completed (delete if not relevant):

• This change follows the Akka.NET API Compatibility Guidelines.
• This change follows the Akka.NET Wire Compatibility Guidelines.
• I have reviewed my own pull request.

Latest v1.4 Benchmarks

BenchmarkDotNet=v0.13.1, OS=Windows 10.0.19044.2006 (21H2)
AMD Ryzen 7 1700, 1 CPU, 16 logical and 8 physical cores
.NET SDK=6.0.201
  [Host]     : .NET 6.0.3 (6.0.322.12309), X64 RyuJIT
  Job-WGPBOC : .NET 6.0.3 (6.0.322.12309), X64 RyuJIT

InvocationCount=1  UnrollFactor=1  

Method	MsgCount	ActorCount	Mean	Error	StdDev	Gen 0	Gen 1	Allocated
PushMsgs	100000	10	1.287 s	0.0067 s	0.0063 s	18000.0000	1000.0000	71 MB
PushMsgs	100000	100	12.818 s	0.1166 s	0.1091 s	176000.0000	1000.0000	689 MB