stream: sync writes don't need drain
Minor fix and performance improvement. Synchronous writes do not require drain.
This can be a significant improvement especially for transform streams that do not perform any async work.
Checklist
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make -j4 test
(UNIX), orvcbuild test
(Windows) passes -
tests and/or benchmarks are included -
documentation is changed or added -
commit message follows commit guidelines
NOTE TO SELF: Sync transform streams should have a no buffer option (i.e. hwm === 0).