Allow optional fields in argument structs

[Raynos]

$ node benchmarks/tchan-server.js --port 9000 --host 127.0.0.1

assert.js:92
  throw new assert.AssertionError({
        ^
AssertionError: no field of an argument struct may be marked optional including targetLocation of bootstrapEater_args
    at ThriftStruct.link (/home/raynos/uber/marketplace-gateway/node_modules/tchannel/node_modules/thriftrw/struct.js:167:20)
    at Thrift.link (/home/raynos/uber/marketplace-gateway/node_modules/tchannel/node_modules/thriftrw/thrift.js:208:32)
    at new Thrift (/home/raynos/uber/marketplace-gateway/node_modules/tchannel/node_modules/thriftrw/thrift.js:73:10)
    at new TChannelAsThrift (/home/raynos/uber/marketplace-gateway/node_modules/tchannel/as/thrift.js:49:17)
    at TChannelAsThrift (/home/raynos/uber/marketplace-gateway/node_modules/tchannel/as/thrift.js:39:16)
    at new BenchServer (/home/raynos/uber/marketplace-gateway/benchmarks/tchan-server.js:30:39)
    at main (/home/raynos/uber/marketplace-gateway/benchmarks/tchan-server.js:167:18)
    at Object.<anonymous> (/home/raynos/uber/marketplace-gateway/benchmarks/tchan-server.js:162:5)
    at Module._compile (module.js:456:26)
    at Object.Module._extensions..js (module.js:474:10)

We spoke a while back about the semantics of arguments structs and believe we came to the conclusion that an arguments struct should be the same as any other struct.

It would be nice to remove this limitation, it would make porting existing HTTP/JSON interfaces to thrift easier since existing HTTP/JSON interfaces can have empty bodies and with optional fields on the main body.