Fix channel buffer size in namespace datastore to use KeysOnlyBufSize

[kevina]

No description provided.

[kevina]

See ipfs/kubo#3376.

I better way would be to use something like make(chan dsq.Result, cap(qr.Next())) but for some reason that cap(qr.Next()) is returning 0 when it should be returning 128 (i.e. dsq.KeysOnlyBufSize) so I manually set the buffer size based on it it was a keys-only query.

[kevina]

Note the test failure is due to the use of go 1.6.3 that no longer appears to be supported with the latest version of go-datastore.

[Kubuxu]

Can you update the travis file to use 1.7.3 and tip.

[kevina]

@Kubuxu

Can you update the travis file to use 1.7.3 and tip.

Done.

[whyrusleeping]

Sweet, LGTM. I think the biggest problem here is that we're trying to use a channel as an iterator and thats just inherently slow. We can look at switching up interfaces later, but the changes being made so far look very promising