Use the stream framework to rebuild indices.

[martinmr]

The stream framework can iterate over the index prefix keys concurrently
whereas currently there's only one thread iterating over the keys.
Changing to the stream framework should provide a speed up over the
current approach.

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This change is 

[martinmr]

The unit tests checking the rebuild process still pass. I manually checked that I could do full text queries after a rebuild with that index.

[gitlw]

Reviewable status: 0 of 2 files reviewed, 1 unresolved discussion (waiting on @manishrjain and @martinmr)

[manishrjain]

Reviewed 1 of 2 files at r1, 1 of 1 files at r2.
Reviewable status: all files reviewed, 2 unresolved discussions (waiting on @martinmr)

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posting/index.go, line 502 at r2 (raw file):

	stream := pstore.NewStreamAt(r.startTs)
	stream.Prefix = r.prefix

stream.LogPrefix = "Indexing..."

[martinmr]

Reviewable status: all files reviewed, 2 unresolved discussions (waiting on @manishrjain and @martinmr)

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posting/index.go, line 502 at r2 (raw file):

Previously, manishrjain (Manish R Jain) wrote…

stream.LogPrefix = "Indexing..."

I'll do this in the next PR to avoid having to wait for another round of teamcity tests. Added a task in asana to avoid forgetting this.