OPTIMIZE: stream: Stop holding a reference on the frontend during stream_free()

OPTIMIZE: stream: Stop holding a reference on the frontend during stream_free()

Holding a reference on the frontend during the whole stream teardown has a
performance impact: all the streams of a frontend atomically increment and
decrement the same counter.

Instead, the frontend capture pools are flushed before the stream connectors
teardown and the remaining pools after it, depending on a flush_pools
flag. That way the frontend is no longer dereferenced after sc_destroy().

Must be backported with the SPOE fix b5db88d9c ("spoe: Let an agent outlive
its applets during a backend deletion"), as far as 3.3.