Support half-open duplex streams

[clue]

React PHP has support for both bidirectional duplex streams and unidirectional half-duplex streams.

• A unidirectional readable stream should close once it can no longer be read.
• A unidirectional writable stream should close once it can no longer be written to.
• A bidirectional duplex stream currently closes once if can either no longer be read or no longer be written to.

We should add an option to allow bidirectional duplex streams to be half-open. In this mode the readable and writable state would be checked independently.

This is particularly useful for some TCP/IP-based protocols where the client sends a FIN message to the server indicating that it will no longer write any data but is still willing to accept incoming data.

[clue]

For the reference, Node.js also has a allowHalfOpen flag which happens to default to true.
It's net module also offers the same options which appears to default to false.

[nick4fake]

Just trapped into a nasty bug with old PHP version (PHP 5.5.10)

If we have write-only stream (new Stream(fopen($file, 'wb), $loop)) then stream_get_contents blocks execution even in non-blocking mode.

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@nick4fake This sounds like a separate issue (which could possibly be related to #46). Can you file a separate ticket and provide some details / a gist so I can look into this? Thanks!

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I still think that this feature makes perfect sense 👍 However, there are currently no immediate plans to build this from my end (no demand at the moment and more important outstanding issues currently), but I suppose we would be really happy to accept PRs 👍