* also make factories more consistent by only offering
FlowGraph.create()
* also remove secondary (edge-based) FlowGraph.Builder DSL
* also improve naming for conversions from Graph to
Source/Flow/BidiFlow/Sink
- 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.
* remove isDemandAvailable
* hide emit from CompletionHandler context
* throw if more than one emit in response to an input
* had to remove the OrderedMerge test/sample because emitting
from CompletionHandler is currently not supported
* FlexiRoute and FlexiMerge might become more capable later, see issue 16753
