Thread safe DCCL

[tsaubergine]

We need a thread-safe DCCL where individual instantiations of the dccl::Codec can safely be concurrently accessed by different threads.

Areas to work on:

• Logger
• FieldCodecManager
• DynamicProtobufManager
• internal::TypeHelper
• internal::MessageStack
• FieldCodecBase
• FieldCodecArithmetic
• DefaultMessageCodec?

Seems like a good deal of structural changes may be required. Perhaps this should be DCCL 3.1?