Don't require seqId for HTTP/2 requests
Don't require seqId for HTTP/2 requests

Summary: Once all H2 Thrift servers (SR proxy and a few others) pick up this change, we'll be able to remove `seqId` from `RequestRpcMetadata`.

Reviewed By: iahs

Differential Revision: D24710010

fbshipit-source-id: 26fe3f9c9052a46e39a0f793f7a56b22bfaca6dc

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