Avoid firing repeated config events while packages are loaded

Refs #4666.

#### Problem
Currently, each scoped settings file or snippets file that is loaded from a package causes expensive events to be emitted on `atom.config`. Emitting these events is expensive because:
* The act of adding scoped settings clears the scoped settings cache
* Callbacks registered via `atom.config::onDidChange` or `::observe` receive the new property value as an argument. Computing the new value requires re-merging the scoped properties with an *empty cache*.

Merging scoped properties recently became more expensive due to deep-merging.

#### Solution

This adds a new method to `Config` called `::transact` which allow multiple config changes to be 'rolled up', firing only one change event.

Currently, I only use `::transact` in `PackageManager` when activating and deactivating packages. I think the snippets package should also use it when loading snippets files from packages.

/cc @benogle @kevinsawicki
Exceptions aren't being caught here; there's only a `finally` block.