Consider reducing unnecessary buffer copies in state operations

[frankbuckley]

There are many places where ByteString.CopyFrom is called which adds overhead/GC pressure, particularly with larger state objects.

Many of these could safely use UnsafeByteOperations.UnsafeWrap to avoid the copy.

Context:

• C# ByteString, CodedInputStream performance improvements desired for messages with large byte array field protocolbuffers/protobuf#4206
• Change ByteString to use memory and support unsafe create without copy protocolbuffers/protobuf#7645

If this is agreeable, I would be happy to provide a PR.

Release Note

RELEASE NOTE: FIX Reduce unnecessary buffer copies in state operations

[WhitWaldo]

Having read the reasoning and seen the benchmarking in protocolbuffers/protobuf#7645 I'm convinced this can be worthwhile, but we're in a code freeze for 1.16 as I ready everything for release. If you could wait to build a PR until after the 1.16 release, I'd certainly accept it then.