Add streaming API

[jpsamaroo]

Adds a spawn_streaming task queue to transform tasks into continuously-executing equivalents that automatically take from inputs streams/channels and put their result to an output stream/channel. Useful for processing tons of individual elements of some large (or infinite) collection.

Todo:

• Migrate streams on first use
• Add per-task input buffering to Stream object
• Add no-allocation ring buffer for process-local put/take to Stream
• Make buffering amount configurable
• Add API for constructing streams based on inferred return type, desired buffer size, and source/destination
• Allow finish_stream(xyz; return=abc) to return custom value (else nothing)
• Upstream MemPool migration changes (Add DRef migration support JuliaData/MemPool.jl#80)
• Add docs
• Add tests
• (Optional) Adapt ring buffer to support server-local put/take (use mmap?)
• (Optional) Make value fetching configurable
• (Optional) Support a waitany-style input stream, taking inputs from multiple tasks
• (Optional) take! from input streams concurrently, and waitall on them before continuing
• (Optional) put! into output streams concurrently, and waitall on them before continuing
• (Optional) Allow using default or previously-cached value if sender not ready
• (Optional) Allow dropping stream values (after timeout, receiver not ready, over-pressured, etc.)
• (Optional) Add utility for tracking stream transfer rates (Add streaming throughput monitor #494)
• (Optional) Add programmable behavior on upstream/downstream Stream closure (how should errors/finishing propagate?)

[JamesWrigley]

Am I correct in thinking that all the necessary items except for tests are complete?

[jpsamaroo]

Generally yes, I think we're pretty close to this being merge-ready. There are some remaining TODOs that I need to finish, but most are reasonably small. I could definitely use help with writing tests - just validating that we can run various kinds of pipelines and that they work across multiple workers would be really useful.