refactored serialization providers.
refactored the serializer registry part of BsonSerializer into DefaultBsonSerializerRegistry.
CR changes.
Refactored DefaultBsonSerializerRegistry and moved the provider related code that was in it to the new TypeMappingSerializationProvider.
moved the location of the global type mapping cache into the global serializer registry.
CSHARP-1102: Remove RequestStart from the public API.