[JENKINS-25514] Fixed lock case in FutureImpl

when some other process is waiting for build in queue.

https://issues.jenkins-ci.org/browse/JENKINS-25514
Test procedute:
1. Start some build using CLI with '-s' option and make sure that this build will stay in queue for some time (offline/busy executor etc)
2. Cancel the build from queue
=>
Expected result: the build is canceled and CLI dropped connection
Result before the fix: the build is canceled and CLI is still waiting without dropping connection.

Strictly speaking this is unnecessary as `start` can be accessed from `setAsCancelled` directly.