Only allow -switch-to-rbr when inspecting replica

[shlomi-noach]

Resubmission of openark#23 from downstream

Fixes #991

gh-ost will fail with error when -switch-to-rbr is provided and when running directly on primary (--allow-on-master).

Low risk

No changes to migration logic, just added command line validation

[timvaillancourt]

👋/:pray: @shlomi-noach, sorry for the delay!

LGTM once this is rebased 👍. Currently can't merge

[shlomi-noach]

The golangci-lint test fails, with what seems like useless output?

2022-08-21T05:54:12.7308332Z ##[group]run golangci-lint
2022-08-21T05:54:12.7312245Z Running [/home/runner/golangci-lint-1.46.2-linux-amd64/golangci-lint run --out-format=github-actions] in [] ...
2022-08-21T05:54:26.8253755Z ##[error]unnecessary trailing newline (whitespace)
2022-08-21T05:54:26.8266486Z ##[error]unnecessary trailing newline (whitespace)
2022-08-21T05:54:26.8267648Z ##[error]unnecessary trailing newline (whitespace)
2022-08-21T05:54:26.8268825Z ##[error]S1011: should replace loop with `result.columns = append(result.columns, columns1.Columns()...)` (gosimple)
2022-08-21T05:54:26.8270059Z ##[error]S1011: should replace loop with `result.columns = append(result.columns, columns2.Columns()...)` (gosimple)

I'm actually unable to find where exactly the linter sees the issue?

[timvaillancourt]

I'm actually unable to find where exactly the linter sees the issue?

@shlomi-noach thanks! Yikes, that output isn't so useful 🤔

This check-annotations page has much more useful output and clicking on each nit jumps to the code (TIL 🚀): https://github.com/github/gh-ost/actions/runs/2897390310. It looks like it's mad about some extra whitespace and 2 x loops that could be simplified to append(something, other...) 👍