update/finish Hydroflow docs

[jhellerstein]

The following was formerly in the TODO.md of the Hydroflow docs.

Hydroflow struct and API

• p1 hydroflow struct and its methods

State & Lifetimes

- discuss stateful ops (`fold_*`, `reduce_*`) and singletons
- `next_stratum` and `next_tick`
- example code (chat?)
- p3 Illustrate the `next_stratum` operator for atomicity (eg Bloom's upsert `<+-` operator)

Debugging

- `inspect`, `for_each` and `null`
- how to use the discrete simulation code

How-Tos and Examples

• p1 Lamport clocks
• p2 Vector clocks
• p2 A partitioned Service
• p2 A replicated Service
• p2 Interfacing with external storage
• p2 Interfacing with external services

Odds and ends taken out of other chapters

• Document the methods on the hydroflow struct -- especially the run methods.

  • The run_tick(), run_stratum(), run(), and run_async() methods provide other ways to control the graph execution.
  • Also run_available() next_stratum() and recv_events are important

• Ensuring that template is up to date with repo

What's currently covered in examples

• Concepts covered
  • cargo generate for templating
  • Hydroflow program specs embedded in Rust
  • Tokio Channels and how to use them in Hydroflow
    • Network sources and sinks (source_stream)
    • Built-in serde (source_stream_serde, dest_sink_serde)
  • Hydroflow syntax: operators, ->, variables, indexing multi-input/output operators
  • running Hydroflow via run_available and run_async
  • Recursion via cyclic dataflow
  • Fixpoints and Strata
  • Template structure: clap, message types
  • source_stdin
  • Messages and demux
  • broadcast pattern
  • the persist operator to store and replay dataflow
  • the defer_signal operator to gate a dataflow
  • bootstrapping pipelines: initialize