Commit e5be3352cf ("database, streaming, messaging: drop
streaming memtables") removed streaming memtables; this removes
the mechanisms to synchronize them: _streaming_flush_gate and
_streaming_flush_phaser. The memory manager for streaming is removed,
and its 10% reserve is evenly distributed between memtables and
general use (e.g. cache).
Note that _streaming_flush_phaser and _streaming_flush_date are
no longer used to syncrhonize anything - the gate is only used
to protect the phaser, and the phaser isn't used for anything.
Closes#7454
5d45662804