Cache known exchange types

Patch provided by GSerjo:
https://groups.google.com/forum/#!topic/rabbitmq-users/Gm7oFTaciKs
merge bug26507 into default
Merge branch 'master' of https://github.com/rabbitmq/rabbitmq-dotnet-client

Conflicts:
	projects/client/RabbitMQ.Client/src/client/api/ExchangeType.cs