Cache result of findGaps() on gapless streams

[jacobtylerwalls]

This approach uses False to distinguish "no gaps" from "force a cache miss" (None), although this has to then be converted back to None for cache hits.

An alternative would be to rewrite the cache (there aren't that many keys) to just delete keys when intending to force cache misses.

Another alternative would be to break BC and have findGaps() return something else, like an empty stream that would still test False.

Closes #1514

[coveralls]

Coverage: 93.053% (+0.001%) from 93.052% when pulling 690dd2c on cache-find-gaps into 2e0ce4e on master.

[mscuthbert]

thanks -- super great.