Exec refactoring

Remember to merge https://github.com/appium/appium/pull/4103 after this.
It is simply stdout/stderr are too big. When we do an page dump it make sense, but case like here:  https://github.com/appium/appium-adb/blob/ffa9e639f167be1347f767b2ff896c0d73b50739/lib/adb.js#L340.

Anyway if we want to stay on the supersafe side it make sense to default it to 512*1024. What would be even safer would be to use spawn for big exec, because if something break 200*1024, it may also reach a point where it breaks 512*1024.