Drop pointer on binary checksum fields

[vytautas-karpavicius]

What changed?
Drop pointer on binary checksum fields within internal types.

Why?
This is one of PRs planned to drop pointers for always required fields in internal types. It will make them more idiomatic and easier to use. These pointer are legacy of Thrift types that were used previously through our codebase. This will also allow safer migration to grpc, as proto primitive types don't have nils by default.

How did you test it?
Passing build and existing tests.

Potential risks

[coveralls]

Coverage increased (+0.05%) to 61.555% when pulling 1fe87e7 on vytautas-karpavicius:drop-ptr-binary-checksum into ece6cac on uber:master.

[Groxx]

Check the comment, but assuming that's fine: LGTM

[Groxx]

Broad Q on this one: what happens for old clients that don't send this field? or have they been cut off?
And how is this stored, could we be inserting "" for them now, or no? (which isn't really wrong, but maybe different / possibly an issue for edge cases?)

[vytautas-karpavicius]

Bad binaries are checked on PollForDecisionTask request. If client don't send this field (nil) we treat it as empty string and pass it to the check function. It then checks against domain config for bad binaries.

So, client that do not set this field will be allowed. Unless "" binary is explicitly configured as bad binary, then it would be denied. It seems, that empty binary is valid, I see no validations checks for that.