Execute prep commands only once if there were many file changes during previous commands execution

[pellared]

Easy repro steps:

1. Clone https://github.com/golang-templates/seed
2. Run modd
3. Open foo.go or foo_test.go
4. Make some file changes e.g. add space and save the file
5. Execute the previous step very fast a lot of times e.g. 30 times
6. Notice that modd executes the build 30 times

Tested with modd v0.8 on golang:1.14.2 Docker container.

I would prefer if modd would execute prep commands only once if there were many file changes during previous commands execution.

I think you can add a state machine to modd. E.g.

1. State "idle - waiting for file changes" - it watches for file changes. If a file is changed then you go to the next state.
2. State "executing prep - waiting for file changes" - start executing the prep. When the prep has finished then go to state 1. If a file changed during prep execution go to next state.
3. State "executing prep - next prep execution queued". When the prep finishes, then just go to the previous step (rerun the prep and wait for new file changes).

Disclaimer, I had not time to look at modd source code.

Described also under #55