CSHARP-1092: Optimize writing BSON arrays by caching UTF8 element names for frequently used index values.
CSHARP-1092: Added tests for ArrayElementNameAccelerator.
CSHARP-1092: Added a missing test for ArrayElementNameAccelerator.
refactored serialization providers.
refactored the serializer registry part of BsonSerializer into DefaultBsonSerializerRegistry.
CR changes.