Switch opcode data type from normal to record constructors

Currently, the OpCode definitions are using normal constructors:

  data OpCode = OpTestDelay Double Bool [String]
              | OpInstanceFailover String Bool (Maybe String)
              …

While this works for a few opcodes, it becomes unwieldy when dealing
with a bigger number of opcode definitions and/or with opcodes having
many fields.

This patch changes the opcodes to record-based constructors, so that
we get for free accessor functions:

  data OpCode
    = OpTestDelay {
        opDuration :: Double,
        opOnMaster :: Bool,
        opOnNodes :: [String]
      }
      | OpInstanceFailover {
         opInstanceName :: String,
         opIgnoreConsistency :: Bool,
         opTargetNode :: Maybe String
      }
      …

Signed-off-by: Iustin Pop <iustin@google.com>
Reviewed-by: Adeodato Simo <dato@google.com>