Optimize single BlockingObservable operations

Backport from 1.x to 0.20.x of #1635
Merge pull request #1637 from benjchristensen/backport-blockingobservable-single-to-0_20

Optimize single BlockingObservable operations
[Gradle Release Plugin] - pre tag commit:  '0.20.2'.
[Gradle Release Plugin] - new version commit:  '0.20.3-SNAPSHOT'.
0.20.2