Revert "Reuse rootReactor if possible for new Promises unless ComposableSpec.fork(true) has been specified or no environment has been provided"

This reverts commit 97cda103e1df1da210c0e338ba7de5537e3eed9b.

Conflicts:
	reactor-benchmark/src/test/java/reactor/core/ComposableThroughputTests.java
	reactor-core/src/main/java/reactor/event/registry/CachingRegistry.java
Revert "Reuse rootReactor if possible for new Promises unless ComposableSpec.fork(true) has been specified or no environment has been provided"

This reverts commit 97cda103e1df1da210c0e338ba7de5537e3eed9b.

Conflicts:
	reactor-benchmark/src/test/java/reactor/core/ComposableThroughputTests.java
	reactor-core/src/main/java/reactor/event/registry/CachingRegistry.java
Merging upstream
Added zero-copy List implementation and use it in the Registry cache to reduce GC pressure of creating lots of Lists to hold cached Registration objects.