Support setting buffer size to null (infinite)

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This has no effect by default and can be triggered by setting bufferSize to null.

This can result in huge performance improvements without causing any memory issues if the buffer size is limited by the underlying OS anyway (such as when reading from a TCP/IP socket).

Also, this can be used to avoid some SSL/TLS hackery in our socket-client component: https://github.com/reactphp/socket-client/blob/f905ec2a576b1242b10d5c1dbd2f0eb4fcea7f0c/src/SecureStream.php#L44

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I've just rebased this to resolve the merge conflict introduced by #39

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I've just rebased this to resolve the merge conflict introduced by #40

[WyriHaximus]

LGTM

[rybakit]

FYI: facebook/hhvm#6921.

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FYI: facebook/hhvm#6921.

Thanks for link 👍 I've updated the documentation to be a bit clearer about what null means.

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