[UDP] Resend inside pending lock

Fixes #61
Don't lock message constructors when not needed
Now requiring .NET 4.5
Drop windows phone 7 support
ISerializationContext now provides all protocols

Completes #57. Additionally, MessageSerializer now maintains an instance
of SerializationContext avoiding having to create a new instance for every
message.
De-inline BufferValueWriter.EnsureAdditionalCapacity

Additionally, make it public.
Tiny optimization on GetBytes()