document memoryview usage, minor frame_writer.write_frame refactor

[pawl]

I was looking at more py-amqp code and found a really interesting usage of memoryview in frame_writer's buffer_store. I added some comments explaining why it's used.

I also moved some variables in frame_writer's write_frame closer to where they're actually used. The buffer_store isn't used by write_frame when it ends up in the if bigbody: block, so I moved the code related to the buffer_store into the else:.

[pawl]

The integration tests failed to pull the rabbitmq image from dockerhub:

Step 1/6 : FROM rabbitmq:latest
Head "https://registry-1.docker.io/v2/library/rabbitmq/manifests/latest": received unexpected HTTP status: 503 Service Unavailable
Error: Process completed with exit code 1.

I think they'll pass if the tests are run again later.

[auvipy]

please make sure unit & integration tests are added or updated to avoid regression

[auvipy]

errors
0
code	"UNAUTHORIZED"
message	"authentication required"
detail
0
Type	"repository"
Class	""
Name	"library/rabbitmq"
Action	"pull"

[auvipy]

the unit tests look good, can you please try to add some integration test as well? besides i want to read the links thoroughly and want others try the open patches so that the don't break

[pawl]

@auvipy I added that integration test here: 98b8bbc