Optimized isUnsubscribed check

[akarnokd]

Extracted isUnsubscribed optimization from #2603.

Benchmark                                   (size)   Mode   Samples          1.x        2603    
r.s.ComputationSchedulerPerf.observeOn           1  thrpt         5   104110.926  104707.286
r.s.ComputationSchedulerPerf.observeOn          10  thrpt         5   100723.402  105825.148
r.s.ComputationSchedulerPerf.observeOn         100  thrpt         5    24609.763   65571.461
r.s.ComputationSchedulerPerf.observeOn        1000  thrpt         5     3212.434   13020.027
r.s.ComputationSchedulerPerf.observeOn       10000  thrpt         5      955.002    1555.493  
r.s.ComputationSchedulerPerf.observeOn      100000  thrpt         5       96.628     160.218  
r.s.ComputationSchedulerPerf.observeOn     1000000  thrpt         5        9.508      16.559  
r.s.ComputationSchedulerPerf.subscribeOn         1  thrpt         5   114212.000  114485.516  
r.s.ComputationSchedulerPerf.subscribeOn        10  thrpt         5   112376.809  112270.024  
r.s.ComputationSchedulerPerf.subscribeOn       100  thrpt         5    88433.002  104240.739  
r.s.ComputationSchedulerPerf.subscribeOn      1000  thrpt         5    31503.640   64446.984  
r.s.ComputationSchedulerPerf.subscribeOn     10000  thrpt         5     3932.988    8200.048
r.s.ComputationSchedulerPerf.subscribeOn    100000  thrpt         5      437.626    1439.069
r.s.ComputationSchedulerPerf.subscribeOn   1000000  thrpt         5       43.104     146.385

[benjchristensen]

Nice improvement.