Hooks

[shlomi-noach]

Hooks

Storyline: #62

This PR introduces hooks, processes to be executed by gh-ost at particular points of interest (e.g. startup, validated, starting postpone, success, status etc.)

• 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

[shlomi-noach]

• Once concurrent row-count #196 is merged, we need to support on-rowcount-complete as well

[pbitty]

I think it would be helpful to pipe hook output (stdout & stderr) to gh-ost's standard output, especially for troubleshooting hook errors.

This might be out of scope of this PR. If so, and you are interested in the idea, I am happy to submit a PR once this is merged.

[shlomi-noach]

Good point. If this is to be done, it will be done in this PR.
Is there any sense in having this configurable?

[pbitty]

I lean towards not having it configurable until someone needs it. An operator can simply choose not to output anything from the hook executable.

[shlomi-noach]

hook stdout & stderr are now written onto gh-ost's stderr.

[shlomi-noach]

Insofar this is looking good in testing. Merging, and will iterate if need be.

[shlomi-noach]

Documentation is on https://github.com/github/gh-ost/blob/master/doc/hooks.md