[Reactive] Reduce the overhead of flatMapIterable + minor fixes

[akarnokd]

The flatMapIterable was underperforming with some short inner collections in benchmarks. The following changes have been applied:

• Use an inlined SPSC queue instead of the ConcurrentLinkedQueue.
• Remove the cancel check between the mapping to the iterator and the first hasNext test
• Disconnect the upstream upon termination so that request on a prefetch boundary becomes no-op (See this discussion.)
• Add a dedicated cross-mapping benchmark measuring flatMapIterable.

Benchmark

(i7 4770K, Windows 7 x64, JDK 13b29)

ShakespearePlaysScrabbleWithHelidonReactiveOpt before  147  34,605 ± 0,210  ms/op
ShakespearePlaysScrabbleWithHelidonReactiveOpt after   153  33,225 ± 0,183  ms/op