Drop, then add same column

[shlomi-noach]

fixes #384

A migration that runs drop column c, ..., add column c now understands the before-c and after-c are different columns, and does not attempt to copy c's data into the ghost table.

• contributed code is using same conventions as original code
• code is formatted via gofmt (please avoid goimports)
• code is built via ./build.sh
• code is tested via ./test.sh

cc @github/platform-data @ikewalker