applyChange emits lots of events when calling `replace` about text-buffer

[maxbrunsfeld]

@nn2242 You're absolutely right; this is a major problem with the current API for observing changes. Unfortunately, it'll take some time to move to a new API. We can't simply emit fewer change events, because the contract of TextBuffer::onDidChange, which many packages rely upon, is that the events it emits fully describe every change that happens to the buffer.

For backwards-compatibility, we need to introduce a new mechanism for observing changes to the buffer. We're thinking of calling it ::onDidTransact. This method would call its callback at the end of every transaction, and it will need to pass an object that describes all of the changes that occurred in the Transaction. We're thinking of using the recently-introduced Patch object to represent the set of changes.

Once we introduce this API, we'll need to migrate all of our packages to use it instead of ::onDidChange.

We haven't started implementing this, but it is definitely on our roadmap for Atom's performance.