Added flags support to grpc_op.

Which includes its propagation to grpc_ioreq and validation mechanisms for checking that only known bits are set ot in).
Also added an internal flag (GRPC_WRITE_INTERNAL_COMPRESS) mask for its use in signaling compressed messages.
diff --git a/test/core/end2end/tests/request_with_payload.c b/test/core/end2end/tests/request_with_payload.c
index ea4bb2c..bf5aa14 100644
--- a/test/core/end2end/tests/request_with_payload.c
+++ b/test/core/end2end/tests/request_with_payload.c
@@ -132,20 +132,25 @@
   op = ops;
   op->op = GRPC_OP_SEND_INITIAL_METADATA;
   op->data.send_initial_metadata.count = 0;
+  op->flags = 0;
   op++;
   op->op = GRPC_OP_SEND_MESSAGE;
   op->data.send_message = request_payload;
+  op->flags = 0;
   op++;
   op->op = GRPC_OP_SEND_CLOSE_FROM_CLIENT;
+  op->flags = 0;
   op++;
   op->op = GRPC_OP_RECV_INITIAL_METADATA;
   op->data.recv_initial_metadata = &initial_metadata_recv;
+  op->flags = 0;
   op++;
   op->op = GRPC_OP_RECV_STATUS_ON_CLIENT;
   op->data.recv_status_on_client.trailing_metadata = &trailing_metadata_recv;
   op->data.recv_status_on_client.status = &status;
   op->data.recv_status_on_client.status_details = &details;
   op->data.recv_status_on_client.status_details_capacity = &details_capacity;
+  op->flags = 0;
   op++;
   GPR_ASSERT(GRPC_CALL_OK == grpc_call_start_batch(c, ops, op - ops, tag(1)));
 
@@ -159,9 +164,11 @@
   op = ops;
   op->op = GRPC_OP_SEND_INITIAL_METADATA;
   op->data.send_initial_metadata.count = 0;
+  op->flags = 0;
   op++;
   op->op = GRPC_OP_RECV_MESSAGE;
   op->data.recv_message = &request_payload_recv;
+  op->flags = 0;
   op++;
   GPR_ASSERT(GRPC_CALL_OK == grpc_call_start_batch(s, ops, op - ops, tag(102)));
 
@@ -171,11 +178,13 @@
   op = ops;
   op->op = GRPC_OP_RECV_CLOSE_ON_SERVER;
   op->data.recv_close_on_server.cancelled = &was_cancelled;
+  op->flags = 0;
   op++;
   op->op = GRPC_OP_SEND_STATUS_FROM_SERVER;
   op->data.send_status_from_server.trailing_metadata_count = 0;
   op->data.send_status_from_server.status = GRPC_STATUS_OK;
   op->data.send_status_from_server.status_details = "xyz";
+  op->flags = 0;
   op++;
   GPR_ASSERT(GRPC_CALL_OK == grpc_call_start_batch(s, ops, op - ops, tag(103)));