ref: Change the API of validators to accept *context.T
instead of security.Call.

Also we remove the Context() accessor from the security.Call and
add it back to rpc.ServerCall.  This is in preparation to split
the ServerCall and security.Call interfaces.

MultiPart: 2/3

Change-Id: Ia41e4a60a3b995ddf349db39fc154cf25d5241bc
7 files changed