build: Add back `SEASTAR_SHUFFLE_TASK_QUEUE`
build: Add back `SEASTAR_SHUFFLE_TASK_QUEUE`

`SEASTAR_SHUFFLE_TASK_QUEUE` was previously defined for debug builds,
but this feature was sadly lost in the CMake transition.

Fixes #601

Tests: unit (debug), dist (debug)
Signed-off-by: Jesse Haber-Kucharsky <526eadea4792054b99d5fa5dd6be9acdd76c5418@scylladb.com>
Message-Id: <ac6494b3c35dec1f724e96c42f2df77721967bee.1549983450.git.526eadea4792054b99d5fa5dd6be9acdd76c5418@scylladb.com>

syuu1228/seastar,syuu1228/seastar,syuu1228/seastar,scylladb/seastar,scylladb/seastar,avikivity/seastar,cloudius-systems/seastar,avikivity/seastar,cloudius-systems/seastar,scylladb/seastar,avikivity/seastar,dreamsxin/seastar,cloudius-systems/seastar,dreamsxin/seastar,dreamsxin/seastar