Remove allocations for AmqpMap enumeration by storing the Dictionary …

[Stabzs]

Addresses https://github.com/Azure/azure-amqp/issues/246

Remove allocations for AmqpMap enumeration by storing the Dictionary as a concrete type and adding an explicit concrete GetEnumerator method.

Reduce allocations for AmqpBufferedMessage by omitting ToArray() call when we already have a concrete list.

Reduce allocations for AmqpDataMessage by presizing segmentList capacity increases when dataList implements ICollection.

[Stabzs]

@microsoft-github-policy-service agree