Performance: Request header improvement

[j82w]

The top 3 methods in the profile shows the header lookups. To avoid this overhead the INameValueCollection interface should add all the headers properties required by TransportSerialization BuildRequest. Then the TransportSerialization logic can be converted to directly reference the property and avoid the overhead of doing lookups by name for all the headers.

The headers are shared by multiple requests for strong and bounded consistency. It is not safe to iterate the headers as 1 thread might modify it while it's being iterated which results in an exception.

TransportSerialization code: https://msdata.visualstudio.com/_git/CosmosDB?path=/Product/Microsoft.Azure.Documents/SharedFiles/Rntbd2/TransportSerialization.cs&version=GBmaster&_a=contents