reuse the Newtonsoft.Json serializer

[Aaronontheweb]

Rather than call JsonConvert on each instance, which creates a new JsonSerializer instance internally - let's just re-use the one created when the NewtonsoftJsonSerializer is instantiated.

Noticed this while working on DU serialization.

Before

BenchmarkDotNet=v0.13.1, OS=Windows 10.0.19041.1110 (2004/May2020Update/20H1)
AMD Ryzen 7 1700, 1 CPU, 16 logical and 8 physical cores
.NET SDK=5.0.302
  [Host]     : .NET Core 3.1.17 (CoreCLR 4.700.21.31506, CoreFX 4.700.21.31502), X64 RyuJIT
  DefaultJob : .NET Core 3.1.17 (CoreCLR 4.700.21.31506, CoreFX 4.700.21.31502), X64 RyuJIT

Method	Mean	Error	StdDev	Gen 0	Allocated
Json_serializer_message_roundtrip	27.83 μs	0.167 μs	0.140 μs	3.2000	13 KB

After

BenchmarkDotNet=v0.13.1, OS=Windows 10.0.19041.1110 (2004/May2020Update/20H1)
AMD Ryzen 7 1700, 1 CPU, 16 logical and 8 physical cores
.NET SDK=5.0.302
  [Host]     : .NET Core 3.1.17 (CoreCLR 4.700.21.31506, CoreFX 4.700.21.31502), X64 RyuJIT
  DefaultJob : .NET Core 3.1.17 (CoreCLR 4.700.21.31506, CoreFX 4.700.21.31502), X64 RyuJIT

Method	Mean	Error	StdDev	Gen 0	Allocated
Json_serializer_message_roundtrip	26.36 μs	0.125 μs	0.104 μs	3.0000	13 KB

Looks like it helped a small amount (looking at the Gen 0 numbers)

[Aaronontheweb]

Added this when I was looking at DU handling for #5196

[Aaronontheweb]

Akka.DistributedData.Tests.Serialization.ReplicatedDataSerializerSpec.ReplicatedDataSerializer_should_serialize_GSet

Failed on all platforms - which makes me suspicious

[to11mtm]

There are thread hazards around re-using a JsonSerializer with certain configurations, IIRC specifically around TypeName resolution and/or Preserving object references. When I did #4929 I found any attempts to re-use the serializer would result in a lot of red on my unit tests. There's a PR to fix in Json.NET but it has been in limbo for some time..

[Aaronontheweb]

There are thread hazards around re-using a JsonSerializer with certain configurations, IIRC specifically around TypeName resolution and/or Preserving object references. When I did #4929 I found any attempts to re-use the serializer would result in a lot of red on my unit tests. There's a PR to fix in Json.NET but it has been in limbo for some time..

That's great to know @to11mtm - I'll de-milestone this PR and mark it as "on hold" then. To be revisited later.