set _ordered = true the default for images.fromList()

[d-v-b]

When creating an images object using the fromList method, the underlying bolt.array object appears to have _ordered == False, which results in any rdd operation on that array requiring a sortByKey, which isn't so great when the rdd is actually ordered in the first place. The default value for _ordered should probably be True in this case.

[freeman-lab]

@d-v-b great idea! Was just discussing with @jwittenbach that this alone might be responsible for a few of the performance regressions we've been seeing.

[freeman-lab]

@jwittenbach is doing some large-scale testing now to confirm

[jwittenbach]

Closed by #339

[d-v-b]

great, thanks!