!str,htc remove FlexiMerge/Route

- replace all occurrences with equivalent GraphStage implementations

This commit introduces a mini-DSL for GraphStage that allows emitting or
reading multiple elements to/from a port with one statement, installing
stateful handlers on the port to make it work. The emitting side allows
stacked continuations, meaning that while an emit() is ongoing (waiting
for demand) another one can be added to the queue; this allows
convenient formualation of merge-type stages.

akka-docs-dev/rst/java/stream-flows-and-basics.rst

@@ -247,5 +247,5 @@ such as ``Zip`` however *do guarantee* their outputs order, as each output eleme
 been signalled already – thus the ordering in the case of zipping is defined by this property.
 
 If you find yourself in need of fine grained control over order of emitted elements in fan-in
-scenarios consider using :class:`MergePreferred` or :class:`FlexiMerge` – which gives you full control over how the
+scenarios consider using :class:`MergePreferred` or :class:`GraphStage` – which gives you full control over how the
 merge is performed.