Introduce HermitObservable

A transformation on an Observable that turns it into a hermit that will
only allow a single subscriber at any given point in time. The resulting
HermitObservable will emit an onError on any Subscriber that tries to
connect while another subscriber is still active.

Change-Id: Ic6d78d3c3f385ccb6124f31342ef726eb200121f
Signed-off-by: Galo Navarro <galo@midokura.com>
Merge "Introduce HermitObservable"