* Start splitting docs up into internals.rst / api.rst
* Docs for lots more of econtext.core.
* Get rid of _update_stream() and has_output(), replace with individual
functions called as state changes.
* Add Broker.on_thread() and remove Stream._lock: simply call
on_thread() to ensure buffer management is linearized.
* Rename read_side/write_side to receive_side/transmit_side like event
handler names.
* Clean up some more repr / debug logs.
* Move handle cleanup to Context.on_shutdown where it belongs.
* Make wake() a noop when called from broker thread.
* Replace graceful_count crap with Side.graceful attribute, add
Broker.keep_alive() to check whether any registered readers want to
be kept alive for graceful shutdown() or any child contexts with a
connected stream exist.
* Make master.Broker timeout slightly longer than slave broker.
* Add generic on_thread() to allow running code on the IO thread.
* Use TLS to track whether importer is currently running. Avoids
needing to maintain an ignore stack.
* Print more debugging around cases where Importer skips a module.
* If a module is part of a package, import the package and examine its
__loader__. If we are not the loader, refuse to load it.
David Wilson