Timeout mode for ostream / istream

[malachib]

Distant relative of #25 , but different

There appears no getting around the blocking nature of ostream / istream. Somewhat baked into the design, so we need a timeout provision. A crude (and probably outdated and broken) polling one exists (see FEATURE_ESTD_OSTREAM_TIMEOUT)

Augmenting aforementioned crude method, we also want a signaling mechanism. Something like FreeRTOS event group indicating more or less CTS, RTS,.

Noteworthy is underlying streambufs appear 100% happy as nonblocking and can stay that way.

[malachib]

A lot of progress here, but pausing because:

1. It's a real rabbit hole
2. 80%+ of this behavior can be approximated by adding a blocking mode (keeping non blocking mode too) to streambufs

Will resume when the true power of signaling comes to fore, which is:

1. Opportunity for streambuf-owned bipbuffer to participate in zero-copy scenarios, quite convenient
2. Finer grained control of timeouts
3. Potentially more optimal sleeping/blocking of thread vs a spinwait