schemawatch: Refresh Watch method to return a notify.Var

[bobvawter]

This change updates the Watcher.Watch() method to return a notify.Var to make it consistent with other sources of asynchronous updates. This will make a follow-on change to cache the target's schema cleaner to implement.

X-Ref: #902

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This change is 

[BramGruneir]

Reviewed 5 of 5 files at r1, all commit messages.
Reviewable status: all files reviewed, 1 unresolved discussion (waiting on @bobvawter and @sravotto)

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internal/target/schemawatch/watcher.go line 89 at r1 (raw file):

// Get implements types.Watcher.
func (w *watcher) Get() *types.SchemaData {
	ret, _ := w.data.Get()

please add comments as to why the return values are not needed