Pre-allocate, check, and consume method, scheme, and content-type headers at

the server.

Verify that method, scheme, and te trailers are present, and error out if not.
Complain if content-type doesn't match the format, but don't error out.

This currently, for now, blindly allows all three schemes (grpc, http, https)
without verification, although that will change once the C implementation
finishes switching to http/https.

Cloned from CL 82558661 by 'g4 patch'.
Original change by klempner@klempner:grpc_metadata:808:citc on 2014/12/19 18:41:47.
	Change on 2015/01/12 by ctiller <ctiller@google.com>
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Created by MOE: http://code.google.com/p/moe-java
MOE_MIGRATED_REVID=83785251